Axiom (5)
I believe the answer is:
maxim
'axiom' is the definition.
(I know that axiom can be written as maxim)
This is all the clue.
(Other definitions for maxim that I've seen before include "Proverb, aphorism" , "Saying - inventor of machine gun" , "Pithy statement of a general truth" , "- Gorky, Russian writer" , "Principle-first machine gun" .)