OED: Adverb and adjective phrase.
1. A phrase used to characterize reasoning or arguing from causes to effects, from abstract notions to their conditions or consequences, from propositions or assumed axioms (and not from experience); deductive; deductively.
2. Hence loosely: Previous to any special examination, presumptively, in accordance with one's previous knowledge or prepossessions.
3. By some metaphysicians used for: Prior to experience; innate in the mind.
(OED, 2nd Ed, accessed through U of MN Libraries)
Taubes: “The second was that ‘there is no a priori reason why this balance should be maintained by control of appetite alone, since it depends as much on calorie expenditure as on calorie intake’” (298).
Sentence: A priori knowledge for the physics problem included the fact that all matter has mass.