Axiom (5)
I believe the answer is:
maxim
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">axiom</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that axiom can be written as maxim)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)