Definition a priori: An a priori argument is one where certain basic principles are assumed to be true. Therefore, it is not necessary to use empirical evidence but rely on the axioms being true.
An example of a priori in economics
A firm will produce where MR=MC because we assume that firms are profit maximisers.
However this a priori argument of assuming profit maximisation may not be true in the real world. There are many other examples of business objectives such as sales maximisation.
A big a priori assumption in economics is that consumers are rational. Classical economics has faced difficulties when these assumptions do not always occur. For example, in the real world we see examples of irrational behaviour.