A device or circuit that generates an AC signal at a certain frequency.
An oscillator is a device or circuit that generates an AC signal at a certain frequency.
One frequent use of oscillators is to provide a clock for microcontrollers. Crystal oscillators are often used, where the oscillator is internal to the microcontroller, and the crystal has to be connected to two pins.
MSP430 32-kHz Crystal Oscillators, TI Application Report