The definition of a truism is a statement of an obvious truth.
An example of a truism is the statement: "A man's gotta do what a man's gotta do."
a statement the truth of which is obvious or well known; commonplace
A statement that is obviously true or that is often presented as true: “the truism that envy often masquerades as resentment” (John Rawls). See Synonyms at cliché.