The term "a priori" is usually defined as the reaching of a conclusion through logical deduction from general truths as opposed to reasoning from empirical observation of specific cases, but it has shades of meaning that go beyond this typical definition. One of them is made clearer by constrasting the term "a priori" with "ad hoc." Something that is ad hoc is a thing, rule, law, or argument which applies only to a specific case of something. It is a provisional explanation, action or rule that does not apply in all cases. Something that is a priori is a rule, argument, or regulating set of principles that applies in all cases. In other words, something that is a priori is seen as a general truth that is not conditioned by any one particular experience or instance of something. For example, time and space were once viewed as a priori absolutes in the universe. Every single thing that exists has a place in time as well as in space.
Carls Jung's "Modern Man in Search of a Soul" and Wikipedia.