n. a formula that asserts that two expressions have the same value; it is either an identical equation (usually called an identity), which is true for any values of the variables, or a conditional equation, which is only true for certain values of the variables (the roots of the equation). For example, x²- 1 = (x + 1)(x - 1) is an identity, and x²- 1 = 3 is a conditional equation with roots x = ±2.