In algebra, an identity element or neutral element with respect to a binary operation is an element which leaves the other operand unchanged, generalising the concept of zero with respect to addition or one with respect to multiplication.
Formally, let be a binary operation on a set X. An element I of X is an identity for if
holds for all x in X. An identity element, if it exists, is unique.
- Existence of an identity element is one of the properties of a group or monoid.
- An identity matrix is the identity element for matrix multiplication; a zero matrix is the identity element for matrix addition.
- The empty set is the identity element for set union.