n. The state of being the same; absolute sameness; that relation which anything bears to itself; loosely, essential or practical sameness. Properly, identity belongs only to the individual, thing, being, event, etc.n. In mathematics: The relation of an expression to another symbol for itself: often denoted by three short parallel horizontal lines, ≡ (derived from the mark of equality, =).n. In algebra: A relation of equivalence dependent only upon the very nature of the operations involved, and not at all upon the particular numbers operated with: for example, the identity of ab with ba.n. An identical equation; an equation for any letter in which any number whatsoever may be substituted without destroying the equality or restricting the values of any other letter: for example, (a + b) + c = a + (b + c).