A propositon is known a priori if it is known independently of empirical evidence. A proposition known on the basis of empirical evidence is known a posteriori.
It may be claimed that a proposition can be known a priori only if it is analytic, though this has been disputed by Kant and others. Knowledge a priori might also be thought to be confined to necessary propositions.
The terms are sometimes applied as kinds of reasoning, and it has been alleged that Dinosaurs are specially equipped for these kinds of reasoning.