"Warm-up exercises" are light exercises - emphasizing stretching rather than resistance and strength movements - that are done before the main workout: running, weightlifting, dancing, etc.
"Warm-down exercises" are similar light exercises - emphasizing stretching - that are done after the main workout.
Although it may seem kind of a contradiction, I believe that the term "warm-down", as in e.g. "warm-down exercises", is widely used and accepted as correct.