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 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.
• 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.

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