Axiom

“Axiom (noun): A generally or universally accepted truth or incontestable principle; self-evident or fundamental proposition; verity. Adj. axiomatic; adv. axiomatically.

‘He recalled a slang axiom that never had any meaning in college days: ‘Don’t buck the system; you’re likely to gum the works.’ John  O’Hara, Appointment in Samarra”

Excerpted from: Grambs, David. The Random House Dictionary for Writers and Readers. New York: Random House, 1990.