An act of pure reason - without the need for sensory knowledge. An a priori act is a mental act of abstraction that only requires sentience, and not any specific experience. The axioms of existence and identity are necessary implications in any thought, and may be gleaned a priori.
It should be noted that 'a priori' reasoning in its most literal sense is impossible - as far as we know. Cognitive development requires experience.
- Copi, I. M, Cohen, C., (2001), "Introduction to Logic", 11th Edition.