1. An electronic circuit that converts energy from a direct-current source to a periodically varying electric output.

2. The stage of a superheterodyne receiver that generates a radio-frequency signal of the correct frequency to mix with the incoming signal and produce the intermediate-frequency value of the receiver.

3. The stage of a transmitter that generates the carrier frequency of the station or some fraction of the carrier frequency.

Any device (mechanical or electrical) which, in the absence of external forces, can have a periodic back- and-forth motion, the frequency determined by the properties of the oscillator.