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).
For example: “concert A” is often defined as having a pitch of 440 Hz (cycles per second). That would be the pitch of its fundamental, which is also its first harmonic. The second harmonic is at 440 x 2 = was 880 Hz, the third at 440 x 3 = 1320 Hz, the fourth at 440 x = 1760 Hz, etc. The second subharmonic, by contrast, would be at 440 ÷ 2 = 220 Hz, the third subharmonics at 440 ÷ 3 = 146.67 Hz, the fourth at 440 ÷ 4 = 110 Hz, etc.
There are a few instruments based on subharmonic synthesis, including Oskar Sala’s Mixtur Trautonium; also of interest is the East German Subharchord. Doepfer’s A-113 Subharmonic Generator module recreates the subharmonic divider portion of it. This movie demonstrates dividing the fundamental’s frequency, and this movie demonstrates mixing those subharmonics.