The word “Science” is, after all, a word. We talking here about defining a symbo composed of an ordered set of letters ( S, C, I, E, N, C and E ) or a set of sounds, that by convention, correspond to those letters. We cannot be sure that a set of letters or sounds exists in any other language that refers to the same concept. When we ask, “what is science”, we are asking what the concept is that comes to mind when we see the this combination of letters or hear the sounds we associate with them. As you will understand when you consider this carefully, not every person responds to the word by calling up the same idea. A lot of what is written about the philosophy of science is motivated by a heartfelt desire on the part of the writer, not only to explain and clarify, but to standardize the concept generated by each person’s mind upon seeing or hearing the word.

I am not being cynical here. Symbols, including words, depend for their usefulness on their ability to evoke similar ideas or concepts in everyone attending to them. If you choose to read the thoughts of philosophers of science included on this page, you will read rely on their use of explanatory words to allow you to understand what the word “science” calls up in each writer’s mind.

Remember, “SCIENCE” is a word, written or spoken. The word is used as a symbol, a stimulus that points to an idea created by human minds for a purpose. In the definitions by the thinkers here and elsewhere, you will notice some agreement and some disagreement. Most writers would agree, that their concept of science includes a method that requires concise communication through writing, reading, speaking and hearing. Most would add mathematics to that list. It is no wonder so many have taken the time to think about and write about his favored “definition” or “conceptual response” to the word, “science”.

I want to tempt you to consider some other English words that call up highly different responses in the minds of different people. “Know”, “Expensive”, “God”, “Art” ,“Love”, “Spirit”, “Good”, “Polite”, “Sex”, “Freedom”, “Bad”, “Fact”, “Believe”.

Stephen Tuell, 2007

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From: Evolution Library Topic Page
“Science is a way of understanding the world, not a mountain of facts. Before anyone can truly understand scientific information, they must know how science works. Science does not prove anything absolutely -- all scientific ideas are open to revision in the light of new evidence. The process of science, therefore, involves making educated guesses (hypotheses) that are then rigorously and repeatedly tested. For a better understanding of the nature and process of science, check out these links, books, and articles.”

From: Philosophy of Science History
“ Sir Francis Bacon : Bacon's method stressed first of all that the scientist (or in his terms the natural philosopher) should rid himself/herself of preconcieved notions or prejudices in order to become "as a child before nature". Facts or observations should then be collected in oreder to compile "natural and experimental histories". Form this tabulated knowledge, correlations are sought, with rules for distinguishing accidental and essential correlations. By this method the true nature of the phenomenon being studied is revealed.”

David Hume:
Hume is perhaps known for his profoundly sceptical tone. Building on the work of John Locke (1632-1704), he analysed our knowledge of causation, and came to the skeptical conclusion that, without making any additional assumptions, it was impossible to obtain sound knowledge of the "necessary" relationships between sequences of events. Suppose you want to know whether A causes B. You make lots of observations of B following A, and can find no instances of A not being followed by B or being followed by something else. Despite all this, there can never be any guarantee that B must follow A, and will always do so in the future. So even in the best of circumstances, you have no sound knowledge that A causes B.

This presents a bit of a difficulty for any theory of science. In practice we invoke an important assumption to justify our faith in observation and experiment telling us something valuable and reliable about the universe. We assume that nature is uniform. In other words we assume that if B follows A here, today, it will follow A somewhere else tomorrow. However, this assumption cannot be independently validated. It could let us down at any time or in any place.”

Immanuel Kant
" Kant was seriously disturbed by David Hume's radical scepticism. In response he held that Hume placed too much emphasis on raw sense data being all that there is to knowledge. It may start with data from the senses but there is more to it than that. In particular he introduced the idea that while objects in the world "out there" have certain crude properties, the mind of the perceiver also supplies some of the properties of objects. Nothing enters our experience completely free of properties that the perceiving mind supplies. Subsequent research has demonstrated that what Kant claimed of minds, actually goes on in our brains. We are not simple receivers of raw sense data.”

John Herschel:
" He drew a vital distinction between the way things are discovered (context of discovery) and the way they are investigated and established (context of justification). He claimed that the manner in which a theory was arrived at, was irrelevant to its acceptibility as a scientfic theory. One might arrive at a theory by means of a dream or sudden flash of inspiration, as well as by careful and painstaking examination of a phenomenon. It made no difference. However, in judging a theory's adequacy, observations were the key element. “

Karl Popper:
"Perhaps one of his key ideas was that the best scientific theories make the boldest claims and are, at least initially, the easiest to "falsify". It is difficult to underestimate the influence Popper had on Western (particularly British) science.”

Thomas Khun:
" Most scientists, most of the time, work within particular paradigms. This determines, for example, which questions are worth asking. Scientists continue to work within a paradigm, even in the face of evidence that it's incorrect. As these anomalies build up, scientists begin to question their paradigm. But, until there is a viable competitor, they continue to stick with their paradigm. Usually, however, a competitor appears. If this can explain more of the data (including anomalies) more successfully than the old paradigm, then a "revolution" or "paradigm shift" takes place. The old is replaced with the new. Science therefore consist of different phases: normal science (when scientists work within their paradigm) and scientific revolutions (when one paradigm is overthrown for another). Progress in any any area comes about through a series of revolutions.”
“The problem with this view was that it suggested that science wasn't nearly as rational and objective as everyone had assumed. In particular it suggested that theories actually conditioned the way we perceived the world. The same basic data could "look" very different when viewed from within different paradigms. And the notion that scientists were just prepared to live with anomalies, sometimes for long periods of time, until a new paradigm came along, rather than actively searching out better alternatives, seemed not to fit well with actual practice in many fields. Some would argue that Kuhn opened up the way for later attacks on science.”

Imre Lakatos:
" Lakatos drew a distinction between refutation (evidence that counts against a theory) and rejection (deciding that the original theory has to be replaced by a new theory). He also argued that it's not individual theories that are accepted/rejected/refuted, but something more complex which he called a "research program". Any research program contains different elements. In particular he defined two terms: the positive and negative heuristic. The positive heuristic was that part of the program which directed scientists toward fruitful avenues of enquiry. The negative heuristic directed them away from other issues and questions. He also argued that the core propositions of any research program were insulated from being falsified by a "protective belt" of auxiliary hypotheses. That's why research programs could resist the occasional negative result. A successful research program build up more and more of these, protecting the core ever more effectively. However, lots of important negative results eat away at this belt, eventually casting doubt on the core propositions.”

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From: Wikipedia, the free encyclopedia
“Science is reasoned-based analysis of sensation upon our awareness. As such, the scientific method cannot deduce anything about the realm of reality that is beyond what is observable by existing or theoretical means. When a manifestation of our reality previously considered supernatural is understood in the terms of causes and consequences, it acquires a scientific explanation.
Resting on reason and logic, along with other guidelines such as parsimony, scientific theories are formulated and repeatedly tested by analyzing how the collected evidence compares to the theory.”

From: Science definition
“The word science comes from the Latin "scientia," meaning knowledge.
How do we define science? According to Webster's New Collegiate Dictionary, the definition of science is "knowledge attained through study or practice," or "knowledge covering general truths of the operation of general laws, esp. as obtained and tested through scientific method [and] concerned with the physical world."

What does that really mean? Science refers to a system of acquiring knowledge. This system uses observation and experimentation to describe and explain natural phenomena. The term science also refers to the organized body of knowledge people have gained using that system. Less formally, the word science often describes any systematic field of study or the knowledge gained from it.

What is the purpose of science? Perhaps the most general description is that the purpose of science is to produce useful models of reality.
Most scientific investigations use some form of the scientific method. You can find out more about the scientific method here.

Science as defined above is sometimes called pure science to differentiate it from applied science, which is the application of research to human needs. Fields of science are commonly classified along two major lines:
- Natural sciences, the study of the natural world, and
- Social sciences, the systematic study of human behavior and society. “

From: What is Science , American Physical Society
“Science is the systematic enterprise of gathering knowledge about the universe and organizing and condensing that knowledge into testable laws and theories.
The success and credibility of science are anchored in the willingness of scientists to:

Expose their ideas and results to independent testing and replication by others. This requires the open exchange of data, procedures and materials.

Abandon or modify previously accepted conclusions when confronted with more complete or reliable experimental or observational evidence.

Adherence to these principles provides a mechanism for self-correction that is the foundation of the credibility of science.”

What is Science, Dan Berger
“I can see why I found this in the "unanswered questions" queue. If all the words written to answer this question were laid end to end, you probably still wouldn't get a straight answer. But I'm gonna try anyhow...

Probably the most concise definition of science is: "Science is what scientists do."

This is not an evasive answer. In fact, Michael Polanyi, who was a successful physical chemist, defined science as a guild in which masters train apprentices to the point that an apprentice is able to phrase and pursue scientific problems on her/his own. What qualifies as a scientifically interesting problem is then defined by the judgement of practicing scientists. Science is a social construction of scientists, who jealously guard the perceived accuracy of each others' results by constant questioning and correction.

One important criterion is that work be reproducible: others should be able to get the same results given the same procedures. But this is not always possible: perhaps someone (like an animal behaviorist or an astronomer) has recorded a rare event; or, something which appeared in my own work, perhaps a technique is difficult enough that others need expert coaching to reproduce it. And some work is not seen (by the larger community) as important enough to replicate. Time is finite, but of making many [experiments] there is no end, and much study is a weariness of the flesh. -- Ecclesiastes 2:12

A good introduction to Polanyi's analysis of the scientific process may be found in the little book Science, Faith and Society, which is a series of three lectures Polanyi gave in 1946. It's about 100 pages long, and while a junior high school student may find it a little difficult, I think it is well and clearly written.

I would caution you that Polanyi's ideas have been distorted beyond recognition by the more extreme post-modernists, who contend that, because all human knowledge is socially constructed, none of it can have any claim to truth. As long as it's "true for you," it's perfectly OK to claim, without evidence, that the ancient Egyptians built airplanes. No scientist -- not Polanyi, nor Thomas Kuhn, another favorite of post-modernists -- would claim such rubbish.
But what is it that scientists do?
The central effort of the scientist was defined by the 18th-Century botanist Ste phen Hales:
Since we are assured that the all-wise Creator has observed the most exact proportions of number, weight and measure in the make of all things, the most likely way therefore to get any insight into the nature of those parts of the Creation which come within our observation must in all reason be to number, weigh and measure.

And, crudely speaking, that's what scientists do: number, weigh and measure. In fact, one need not even be a genius to be a good scientist as long as one understands one's field and is willing to do the patient drudgework of measurement.

Where one man of supreme genius has invented a method, a thousand lesser men can apply it.
-- Bertrand Russell

But don't scientists make theories about what they find?
Certainly. Theory is the forging of knowledge into understanding, and has be seen as a prosaic process. Einstein said the whole of science is nothing more than a refinement of everyday thinking, and John Polkinghorne has said that his career as a theoretical subatomic physicist was a matter of trying desperately to keep up with experimental results.

Theory has its skeptics, and not everyone thinks its conclusions are always valid; as Karl Popper said, Science may be described as the art of systematic over-simplification.
Someone whom I cannot remember divided scientists into synthesizers, who try to find the common core which explains a variety of facts, and the diversifiers, who take joy from uncovering more and more complex processes. Synthesizers tend to be theorists, and diversifiers experimentalists. Probably the best statement of the diversifiers' point of view was found in Science a while back, in a news story about the discovery of a new subatomic particle predicted by theory:

"It was nice to find the confirmation of this process. But it would have been even better to have found something totally unexpected, so that the theorists would have had to go back and work harder."
-- paraphrase of an experimental physicist

So what is it scientists believe?
Scientists are people of simple faith in the intelligibility of the universe, as Vannevar Bush saw.
"Science has a simple faith, which transcends utility... that it is the privilege of man to learn to understand, and that this is his mission."
Science can be lifted onto such a pedestal that it is seen as the supreme activity of humanity and its only source of truth; this is the basis of logical positivism (here's another view), and of much modern materialism and atheism.

A warning against the deification of science was given by Wyndham Lewis:
"When we say "science" we can either mean any manipulation
of the inventive and organizing power of the human intellect:
or we can mean ... the religion of science, the vulgarized derivative
from this pure activity manipulated by a sort of priestcraft into
a great religious and political weapon."

While some scientists take this inflated view of scientific knowledge, most of them are more humble. For example, Duke University biologist Matt Cartmill recently wrote in Discover magazine,
"If biologists don't want to see the theory of evolution
evicted from public schools because of its religious content,
they need to accept the limitations of science and
stop trying to draw vast, cosmic conclusions from
the plain facts of evolution. Humility isn't just a cardinal
virtue in Christian doctrine; it's also a virtue in the practic
e of science."

So what's the point of science, anyhow?
Scientists love their calling; it's one of the few professions in which you can be paid for having fun. Francis S. Collins, who heads the Human Genome Project, says that
"When something new is revealed ... I experience a feeling of awe
at the realization that humanity now knows something only
God knew before."

Others go further: Steven Weinberg has written,
"The effort to understand the universe is one of the very few things
that lifts human life a little above the level of farce, and gives it some
of the grace of tragedy."

Many utilitarian answers have been given to the question of "what good is it anyhow?"
Few can doubt the importance of technology ("applied science") in their lives, and science is often justified because of the technology it can lead to. Louis Pasteur pointed out that
There does not exist a category of science to which one can give the name applied science.
There are science and the applications of science, bound together as the fruit of the tree which bears it.But more importantly, science is one of the human arts, in which the first qualification is to see the world with wonder in your soul.
To paraphrase Sir Walter Scott,
Breathes there a man with soul so dead, who never unto himself hath said,
Wow! This is really neat stuff!
-- cribbed from The Lay of the Last Minstrel

The scientist lives, even more than the artist,
To see a world in a grain of sand
And a heaven in a wild flower,
Hold infinity in the palm of your hand
And eternity in an hour.
William Blake (1757-1827), Auguries of Innocence “

Other excellent definitions and discussions. Too long to include here:

What Is Science Feynman
What is Science? A Baseline Definition.. Hooker
What is science? by Sun Nordwall
What is Science?

A Tabular History of Scientific Ideas That Challenged Fundamental Notions of the World

An Illustrative Joke:
A carpenter, a school teacher, and scientist were traveling by train through Scotland when they saw a black sheep through the window of the train.
"Aha," said the carpenter with a smile, "I see that Scottish sheep are black."
"Hmm," said the school teacher, "You mean that some Scottish sheep are black."
"No," said the scientist glumly, "All we know is that there is at least one sheep in Scotland, and that at least one side of that one sheep is black."