noun Also equivalency (for defs 1, 2).

- the state or fact of being equivalent; equality in value, force, significance, etc.
- an instance of this; an equivalent.
- Chemistry. the quality of having equal valence.
- Logic, Mathematics.
- Also called material implication.the relation between two propositions such that the second is not false when the first is true.
- Also called material equivalence.the relation between two propositions such that they are either both true or both false.
- the relation between two propositions such that each logically implies the other.

adjective

- (of a logical or mathematical relationship) reflexive, symmetrical, and transitive.

noun

- the state of being equivalent or interchangeable
- maths logic
- the relationship between two statements, each of which implies the other
- Also called: biconditionalthe 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

1540s, from French équivalence, from Medieval Latin aequivalentia, from aequivalentem (see equivalent). Related: Equivalency (1530s).