axiom

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A self-evident or universally recognized truth; a maxim.
• noun An established rule, principle, or law.
• noun A self-evident principle or one that is accepted as true without proof as the basis for argument; a postulate.

from The Century Dictionary.

• noun one of those generalizations of ordinary experience which nobody doubts, and which are soon replaced by scientific formulations, which latter are also, but less properly, termed middle axioms.
• noun A self-evident, undemonstrable, theoretical, and general proposition to which every one who apprehends its meaning must assent.
• noun Any higher proposition, obtained by generalization and induction from the observation of individual instances; the enunciation of a general fact; an empirical law.
• noun In logic, a proposition, whether true or false: a use of the term which originated with Zeno the Stoic.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Logic & Math.) A self-evident and necessary truth, or a proposition whose truth is so evident as first sight that no reasoning or demonstration can make it plainer; a proposition which it is necessary to take for granted; as, “The whole is greater than a part;” “A thing can not, at the same time, be and not be.”
• noun An established principle in some art or science, which, though not a necessary truth, is universally received.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun philosophy A seemingly self-evident or necessary truth which is based on assumption; a principle or proposition which cannot actually be proved or disproved.
• noun mathematics, logic A fundamental theorem that serves as a basis for deduction of other theorems. Examples: "Through a pair of distinct points there passes exactly one straight line", "All right angles are congruent".
• noun An established principle in some artistic practice or science that is universally received.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (logic) a proposition that is not susceptible of proof or disproof; its truth is assumed to be self-evident
• noun a saying that is widely accepted on its own merits