A rule of inference in propositional logic, written as 'Equiv.' for short.

(P≡Q) = {(P→Q) ∧ (Q→P)}
(P≡Q) = {(P∧Q) ∨ (¬P∧¬Q)}

Or in English: saying that "P is the same as Q" is the same as saying either that "anytime that you have P then you will also have Q AND anytime that you have Q then you will have P" or "either both P and Q are true OR neither P nor Q are true."

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