Noun
an assumption that is basic to an argument
Source: WordNetWe unite behind a declaration of self-evident truth. Alex Salmond
In general, a non-logical axiom is not a self-evident truth, but rather a formal logical expression used in deduction to build a mathematical theory. Source: Internet
In formal systems, the word axiom has a special meaning, different from the ordinary meaning of "a self-evident truth". Source: Internet