The set of all possible output values of a function.
(The set of actual output values is called the range.)
In other words, the set of values that the output values lie in.
Example: for the square of the natural numbers N = {1, 2, 3, ...}:
• the domain (input values) is N
• the codomain (possible output values) is N
• the range (actual output values) is {1, 2, 4, 9, 16, ...}