When discussing harmonics, we usually talk about a fundamental harmonic that is at the same pitch as the sound being played, and the a series of higher harmonics added on top of that which gives a sound its unique character. The relationship between the fundamental and a higher harmonic is a number the fundamental is multiplied by (often an integer) to get the higher harmonic.
However, it’s also possible to find circuits that divide instead of multiply the fundamental harmonic to produce lower frequencies, and therefore subharmonics. The most common is an octave divider or sub bass circuit that divides creates a subharmonic by dividing the fundamental by 2 (some can also create a subharmonic two octaves below the fundamental by dividing it by 4).
There have been attempts to create a synthesis technique based on blending subharmonics rather than higher harmonics – kind of the opposite of additive synthesis, which is based on mixing higher harmonics. The most famous is the Mixtur-Trautonium created by Oskar Sala; also of interest is the East German Subharchord.