A seemingly self-evident or necessary truth which is based on assumption; a principle or proposition which cannot actually be proved or disproved.
A fundamental assumption that serves as a basis for deduction of theorems; a postulate (sometimes distinguished from postulates as being universally applicable, whereas postulates are particular to a certain science or context).
An established principle in some artistic practice or science that is universally received.
A word used as a motto, as expressive of a principle, belief, or rule of action; a rallying cry.
A prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies.