1. A machine which converts mechanical energy into electrical energy.

In its most common form, a large number of conductors are mounted on an armature which is rotated in a magnetic field produced by field coils.

2. A vacuum-tube oscillator or any other non-rotating device that generates an alternating voltage at a desired frequency when energized with direct-current power or low-frequency alternating-current power.
3. A circuit that generates a desired repetitive or non-repetitive waveform; such as, a pulse generator.