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.