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The set of all possible output values of a function.
(The set of actual output values is called the range.)

Or in other words the set of values that the output values lie in.

Example: if the function is the square of the natural numbers N = {1,2,3,...}, then

* the domain (input values) is N
* the codomain (possible output values) is N
* the range (actual output values) is {0,1,2,4,9,16,...}