If the range of a function is a singleton set then the function is called a constant function.

f : A → B is such that f(a) = b for all a ∈ A, then f is called a constant function.

Let A = {1, 2, 3}, B = {a, b}. If the function f is defined by f(1) = a, f(2) = a, f(3) = a then f is a constant function.

f : R → R, defined by **f(x) = k** is a constant function.