It is an adjective.
Meaning: using facts or principles that are known to be true in order to decide what the probable effects of something will be.It means we use it when something derived by reasoning from some known facts.
Synonyms : theoretical, deduced, deductive, inferred, scientific, postulated, suppositional, self-evident,rational,derivable,presumptive.
a priority - noun
1)The deduction is based on assumptions and not a priori reasoning.
2)People have voted to the party on their a priori knowledge.