a posteriori (adverb, adjective): a Latin phrase. The preposition ab, abbreviated to a before a consonant, means “from.” Posterior is a Latin comparative adjective based on post “after,” perhaps from the Indo- European root apo- “away, off” (off is a native English derivative of that root). Surprisingly, in light of their phonetic differences, both Latin a(b) and post may have descended from the same Indo-European root:when you go off (ab), you move away to a place where you will be afterwards (post). In mathematics or science, when you reason a posteriori, you start with what comes “after,” namely facts or observations, and try to go back to the general principles that are presumed to underlie those specific facts or observations. Compare induction; contrast a priori and deduction. [14]

a priori (adverb, adjective): a Latin phrase. The preposition ab, abbreviated to a before a consonant, means “from.” The Indo-European root may be apo- “away, off.” Prior is a Latin comparative adjective based on pro- “before.” The Indo-European root is per- “forward, before.” When you reason a priori, you start with axioms and postulates that “come before,” and try to find effects, facts, or relationships that necessarily follow. Compare deduction; contrast a posteriori and induction. [14, 165]

abacus, plural abaci (noun), abacist (noun): the Latin word abacus was taken from Greek abax “a slab,” probably from the Hebrew abhaq “dust.”