EQUIVALENCE

also called equivalency, plural equivalences or equivalencies in classical prosody, the principle that one long syllable is equal to two short ones. The principle is used as the basis for substitution in quantitative verse. also called equivalence of propositions in logic and mathematics, the formation of a proposition from two others which are linked by the phrase if, and only if. The equivalence formed from two propositions p and q also may be defined by the statement p is a necessary and sufficient condition for q.