truism is a statement so obviously true that asserting it adds nothing — an undoubted truth, especially one too self-evident to be worth mentioning.
The word almost always carries a note of dismissal. Calling a remark a truism concedes its accuracy while denying it any weight, which is a sharper move than calling it false. "Hard work matters" is not wrong; it is a truism, and the objection is that it does no work in the argument it was offered to support. Neighboring terms sit at different points on the same scale: a cliché is worn out by repetition, a platitude is offered as though it were insight, and a 📝maxim compresses guidance into a line worth remembering.
The form is transparent — true plus the suffix -ism, on the pattern of words naming a characteristic utterance. Its logical cousin, the tautology, is true by its own structure ("it will rain or it will not"), while a truism is true about the world and merely too plain to argue with.
The distinction from an 📝axiom is the useful one. Both are granted without proof, but an axiom is granted deliberately, as the foundation something gets built on; a truism is granted reflexively, and nothing gets built on it.