Plain English Meaning
A small generator attached to the engine that turns the engine's spinning into an electrical signal, which the cockpit gauge then reads as RPM.
Full Definition
A small electrical device coupled to a rotating shaft that produces a voltage proportional to the shaft's rotational speed, used to measure and display engine RPM on a tachometer.
Context Anchor
Seen in discussions of engine instruments, maintenance, and training devices that need to sense or display rotational speed.
Derivation
Tach' is short for tachometer, from the Greek 'tachos' meaning speed. A tach generator, then, is literally a 'speed generator' — a device that generates an electrical signal representing speed of rotation.