dienasgramata:

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.

In logic, a proposition may be a truism even if it is not a tautology, a restatement of a definition, or a theorem derived from axioms that are generally held to be true. In fact, some would say that such analytic propositions should not be regarded as truisms.

In philosophy, a sentence which asserts incomplete truth conditions for a proposition may be regarded as a truism. An example of such a sentence would be: "Under appropriate conditions, the sun rises." Without contextual support — a statement of what those appropriate conditions are — the sentence is true but uncontestable. A statement which is true by definition ("All cats are mammals.") would also be considered a truism.

Often the word is used to disguise the fact that a proposition is really just a half-truth or an opinion, especially in rhetoric.