Literally, Latin for “from what comes before.” (a) Used to describe preexisting (prior) conditions among groups of subjects, especially potential ∗confounding variables. (b) Said of conclusions reached on the basis of reasoning from self-evident propositions—without or before examining facts—or of research that proceeds in a ∗deductive way. Loosely, theoretical. Compare ∗a posteriori.
The expression is often used to describe a conclusion for which someone believes there is no empirical evidence, as in: “One conclusion is as likely to be true as the other, a priori, which is why we need to gather more data to resolve the question.”