In the context of overtones, it is the tone from which all the overtones originate. So, if the fundamental ‘X’ is 100HZ (Cycles per second) then the overtones will be whole number multiples of that tone.

Examples 1x= 100HZ (Fundamental)

2x= 200HZ   (Octave above the fundamental)

16x=1600HZ   (4 octaves above the fundamental)

The number times the fundamental frequency will be the number assigned to that particular overtone that has an equally particular sound that can be recognised. See Overtone Series.