: one of three principles in logic:
1. : a statement (as “a house is a house”) in which the subject and predicate are the same is true
2. : the copula in an identity affirms an existent of which the identity is true
3. : a statement of an identity is the expression of an abstract relation of identity symbolized by a term (as A in “A is A”) that apparently refers in its separate instances to the subject and predicate respectively