Definition of Scientific Terms

An observation or experimental result that has been repeatedly confirmed so that for all practical purposes it is accepted as "true."
Caveat: Truth in science is never final. It is always possible that what is accepted as a fact today may need to be modified or even discarded tomorrow. Though this rarely happens in practice, we should keep our minds open to the possibility that it might. Sometimes it happens that what was perceived as a fact was really just a special case of some more general truth.

A basic truth or assumption.
It is necessary to start somewhere. In science, this often begins with a statement of principle; some assumption on which a hypothesis or even a theory can be based. In this sense, its use is rather like that of an axiom in mathematics. It is preferable to make do with as few of these as possible.
Example: The Equivalence Principle: Einstein's General Theory of Relativity is based on the equivalence of inertial and gravitational mass.

Natural Law
A rule or descriptive generalization about how some aspect of the natural world behaves under stated circumstances.
Example: Newton's First Law of Motion: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

A suppostion about the natural world leading to deductions that can be tested. If the deductions are verified, the hypothesis is provisionally corroborated. If the deductions are incorrect, the original hypothesis is proved false and must be abandoned or modified. Hypotheses can be used to build more complex inferences and explanations (theories).
Example: Galaxies are surrounded by a halo of dark matter.
Motivated by: Galaxy rotation curves are observed to be flat when they should decline if all the mass present were that visible in stars.
Testable deduction: The material that composes the dark matter can be detected by direct means.
This example requires a further hypothesis as to the nature of the dark matter: a particular hypothesized candidate.
e.g., very faint stars (brown dwarfs); a new fundamental particle (WIMPS); very small rocks...

In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.
Example: Einstein's Theory of General Relativity
A theory encompasses a large swath of knowledge, and can be employed in practical applications (for example, Newton's Law of Gravity, a subset of Einstein's General Relativity, is frequently employed to predict the trajectories of particles under the influence of massive objects. In such fashion one can accurately navigate a satellite to another planet or precisely drop bombs on other peoples' heads.)
Ideally, a theory should make predictions that (as with the deductions of hypotheses) can be tested in regimes not yet explored experimentally.
For example, General Relativity predicts the existence of black holes and gravitational waves.

A set of assumptions and concepts that constitute a way of viewing reality.
Generally specific to a particular community or intellectual discipline.
May encompass a number of specific well-tested theories as well as more general knowledge and assumptions, knit into a consistent picture but not necessarily comprising a well-tested theory in and of itself.
Problems with a paradigm may lead to cognitive dissonance.

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