A machine — or, more often now, a small part on a board — that makes something swing back and forth in a steady rhythm. In electrical work, that “something” is voltage or current: an oscillator produces electrical oscillations, a wave that goes up and down again and again, like a heartbeat the rest of the circuit can follow. Radios, computers, and quartz watches all need that steady beat; without an oscillator they would drift. The word lives in engineering more than in the kitchen. In ordinary talk you are more likely to hear the verb: a fan oscillates, a person oscillates between two choices. “Oscillator” is the name for the thing that makes the swing on purpose. People do stretch it a little — a pendulum, even a mood that will not sit still — but if someone says it at work they almost always mean the electrical kind.