• Detuning is when an oscillator is shifted away from the root note.
• Usually an intentional detune is relative to the fundamental frequency; it is consistently an octave or some cents above or below the root.

FM/PM Synths

Phase Modulation synths seem to have more detuning than standard subtractives:

• Phase modulation (the output of other modulators.)
• Octave-relative detuning (as in traditional subtractives.)
• Hertz detuning

Hertz detuning can be seen adding non-relative characters to sound in this video:

• Sytrus From The Ground Up 4 - Hertz and Ratio