British Dictionary definitions for material-equivalence
equivalence
/ɪˈkwɪvələns/
noun
1.
the state of being equivalent or interchangeable
2.
(maths, logic)
the relationship between two statements, each of which implies the other
Also called biconditional. the binary truth-function that takes the value true when both component sentences are true or when both are false, corresponding to English if and only if. Symbol: ≡ or ↔, as in –(p ∧ q) ≡ –p ∨ –q