The synthetic a priori is a term coined by Kantdescribing a type of judgment This was his breakthrough thought.

A synthetic a priori judgment is a judgment presupposed by the possibility of experience. For Kant, for example, causality was a synthetic a priori judgment. If I throw a ball into the air, do I need to have done so to guess what will happen? Kant would say no, whereas David Humewould say yes.