An undoubted or self-evident truth; a statement which is pliantly true; a proposition needing no proof or argument; -- opposed to falsism.

a statement the truth of which is obvious or well known and whose utterance, therefore, seems superfluous; an undoubted or self-evident truth, especially one too obvious for mention.

an obvious truth

a statement most everyone will agree with and therefore, can not be argued against

a statement or principle that over time comes to be accepted as fact

something undeniably true which you can say without any need to justify it. Truisms please people and are, largely, irrelevant. They are a useful tool. See Promising The World.

A truism is a claim that is so obvious or self-evident as to be hardly worth mentioning, except as a reminder or as a rhetorical or literary device.