Harmonic Randomizer

A harmonic randomizer is a setting or a group of settings meant to partially and randomly nudge the harmonics of a synth to randomize its timbre, usually only slightly.

Harmonic Selection
Straightforward; some dials allow for negative values for grouping everything above the setting and randomizing all of those.

Harmonic Amplitude Randomness
This selects the function by which the harmonic randomizes itself. You may additionally get to set how much variety is in that randomization.

The same effect can be achieved with a low-volume LFO following a noise wave attached to the harmonics.