Science

In a Nutshell
Science is the idea that the universe is knowable, and that evidence and reproducible experiment are the best ways of discovering it. (hat-tip to Jonathan on Contrary Brin)

Definition & Terminology
The word "science" generally refers to any of the following:
 * 1) the scientific method: an intellectual endeavor aimed at a rational understanding of reality (also described as "the natural and social world") -- the methodology by which scientific knowledge is derived
 * 2) scientific knowledge: a corpus of currently accepted substantive knowledge
 * 3) the scientific community: the community of scientists, with its mores and its social and economic structure
 * 4) applied science: technology (as in "science has achieved many wonderful things" like "digital watches", "New York, wars, and so on")

("The scientific community" is also sometimes referred to as "the scientific establishment", especially when the speaker is proposing some theory based on principles which have been generally rejected by said establishment.)

For the purposes of discussion on this site, "science" by itself should be taken as referring to definition #1.

Note: a possible addition to the above list:
 * scientific ideal: when it comes to making decisions, finding the truth is more important than any other consideration. This is why questioning everything is important, because only through questioning can you move from where you are in order to get closer to the truth. (This is also why science is fundamentally hostile to religion, because religion depends on ideological protectionism to survive, and you can't question everything while protecting some things from questioning.)

Key Attributes
Science (definition #1) is a methodology aimed at acquiring accurate knowledge about reality, with reason and observation being given primacy over any other methods of discovery.

Science's methodology is characterized, above all else, by the critical spirit: the commitment to the incessant testing of theoretical assertions through observations and/or experimentation, and to revising or discarding those theories where experiment shows them to be inaccurate.

In other words:
 * In order to influence the body of scientific knowledge, an observation must be verifiable. If subsequent testing does not confirm the original observation, then that observation is rejected.
 * In order to be called scientific, a theory must be falsifiable. If there is no conceivable observation which would prove the theory wrong, then it is not falsifiable and it is not scientific.

One corollary of this is fallibilism: the understanding that all of our empirical knowledge is tentative, incomplete, and open to revision in the light of new evidence or cogent new arguments. (It should be understood, though, that the more an existing piece of scientific knowledge has withstood scrutiny, the more devastating must the evidence or argument be in order to succeed in unseating it in any capacity.)

Agreement on what constitutes scientific knowledge is not unanimous in all cases. The most widely-accepted pieces of scientific knowledge are part of an interlocking framework, where each piece reinforces the others, and information from one area of science (e.g. biology) must not contradict information from other areas (e.g. physics, chemistry); if an inaccurate piece of information somehow "got in", it would quickly become apparent that it was inconsistent with countless others. Likewise, when one piece of such widely-accepted scientific knowledge is proven to be false (e.g. that the sun does not revolve around the earth), the implications generally affect other, related pieces, and sometimes go far beyond that one piece; when a key piece of knowledge is overturned, it often leads – like one first stone in a tightly-packed layer finally coming loose – to countless new truths being discovered, which then become part of a new, more accurate and more complete framework.

Science generally proceeds from the assumption that there is an objective reality or truth. Science does not presume that a full understanding of that objective reality can ever be achieved, but works instead from a process of successive approximation, i.e. iteratively improving on our understanding of reality without necessarily ever knowing all of it.

To oversimplify all this just a bit, science has one rule: Everything must be questioned, even this rule.

1
[http://www.physics.nyu.edu/faculty/sokal/pseudoscience_rev.pdf Pseudoscience and Postmodernism: Antagonists or Fellow-Travelers? (PDF)] by Alan Sokal : includes an excellent lay-level overview of the key attributes of science, a similar overview of pseudoscience, and guidelines for distinguishing between them; portions of the explanation above are extensively quoted and paraphrased from that article.

2
...as opposed to postmodernism, which asserts that "truth" only exists within the context of a given social group's reference frame.

3
...as opposed to most (all?) religion, which generally asserts the possession of a set of absolute and/or unchanging truth via divinely-guided writing, ancient scrolls, or other mystical and supernatural means.

Related Articles

 * anti-science
 * religion vs. science
 * evolution by natural selection

Reference / Projects

 * The Science wars were a series of intellectual battles in the 1990s between "postmodernists" and "realists" about the nature of scientific theories; the postmodernists questioned the objectivity of science, and the realists countered that there is such a thing as objective scientific knowledge and accused the postmodernists of having a poor understanding of the subject.
 * Hindsight bias
 * Overcoming Bias:
 * Hindsight bias
 * Hindsight Devalues Science
 * Bad Astronomy is a site which debunks astronomically-related hoax claims
 * Center for Inquiry: "working to promote and defend reason, science, and freedom of inquiry..."
 * Public Library of Science: "nonprofit organization of scientists and physicians committed to making the world's scientific and medical literature a freely available public resource"
 * Essays by Richard Feynman:
 * What Is Science?: one definition is "Science is the belief in the ignorance of experts." "Science alone of all the subjects contains within itself the lesson of the danger of belief in the infallibility of the greatest teachers of the preceding generation."
 * Cargo Cult Science: examples of the "leaning over backwards" (to avoid self-deception) which characterizes true scientific investigation
 * repeats the false claim that Albert Einstein believed in God
 * stub article (as of 2008-05-20)
 * stub article (as of 2008-05-20)
 * stub article (as of 2008-05-20)

Humor

 * 2011-04-09 Dresden Codak (webcomic): how the scientific process can go wrong

Articles

 * 2007-07-27 A prescription for terror: "A substantial number of perpetrators of terrorism are products of a scientific education. Debora MacKenzie asks whether there is a connection and how deep it might go."
 * 2007-07-01 The new age of ignorance by Tim Adams: "We take our young children to science museums, then as they get older we stop. In spite of threats like global warming and avian flu, most adults have very little understanding of how the world works. So, 50 years on from CP Snow's famous 'Two Cultures' essay, is the old divide between arts and sciences deeper than ever?"

Other

 * scientific communication 2.0: a diagram of how scientific knowledge is arrived at and how it reaches the public