1. (Science: logic) Characterising that kind of reasoning which deduces consequences from definitions formed, or principles assumed, or which infers effects from causes previously known; deductive or deductively. The reverse of a posteriori.
3. (Science: philosophy) Applied to knowledge and conceptions assumed, or presupposed, as prior to experience, in order to make experience rational or possible. A priori, that is, form these necessities of the mind or forms of thinking, which, though first revealed to us by experience, must yet have preexisted in order to make experience possible. (Coleridge)
Origin: L. A (ab) – prior former.