An overtone that's a whole number multiple of the fundamental frequency of a string is called a?

harmonic

Explanation:

The frequency of a wave is given by

where

n = Order

when n = 1 it is the fundamental frequency, first harmonic, first partial

n = 2 it is the first overtone, second harmonic, second partial

n = 3 it is the second overtone, third harmonic, third partial

and so on

So, the overtone is the whole number multiple of the fundamental frequency is called a harmonic.