A constant is a mathematical quantity that is not variable. Constants occur in almost all polynomials, in the form of either coefficients or addends. Thus in ax³ + bx² + cx + d, each of a, b, c, d is a constant; should the expression be set to zero then the values of the constants determine the values of x for which the equation holes; should the expression be a function then the constants determine the values of x for which f (x) is defined. Constant addends are usually symbolized by c, C, k, or K, though this is by no means universal.

Some constants are naturally occurring. One such as pi, which is of considerable importance in mensuration; another is e. Physical quantities whose scalar is constant, such as c, the speed of light, or G, the gravitational constant, are termed physical constants.