Equivalence (EQV) means equivalence. It exists when one model is equivalent to another and vice versa.
Equivalence can also refer to mathematical or logical statements and is used to simplify dual arithmetic. Equivalence is present in the case of equal states, it is equivalence that can refer to signals, circuits or even logical states. In terms of logics, equivalence is an if-equal relation. If this is fulfilled, for example if both inputs have "1", then the statement is correct, if it is not fulfilled, because one input is "0" and the other "1", then the statement is false.