From CS2800 wiki

The codomain of a function [math]f [/math] is part of a function definition; it broadly describes the outputs from the function, without necessarily describing the set of possible outputs exactly. For example, you might say a function outputs "numbers", without checking whether it can output every possible number; you would then specify the codomain as the set of numbers.

This is in contrast with the image, which is the set of values that are actually output.