a principle, rule, or maxim widely accepted on its intrinsic merit; a generally recognized truth
All the nice people were poor; at least, that was a general axiom, the best of the rich being poor in spirit — Muriel Spark
a proposition regarded as a self-evident truth
something taken as true for the sake of argument; a postulate
[via Latin from Greek axiōma honour, from axioun to think worthy, from axios worth, worthy]