A function generator whose output is a  periodic waveform. 

An oscillator operating at sub-audio frequencies is often refered to as a low-frequency-oscillator (LFO). 

Analog oscillators generate an electrical function or waveform. These are typically approximations of simple trigonometric shapes such as sinusoidal, square, triangular, and pulse.  More sophisticated synthesizers will permit the user to mix different waveforms and to process them in various ways such as modulating the pulse width or altering the shape in some other way, Digital oscillators can generate these same shapes using mathematical descriptions or look up shapes from a table and generate arbitrarily complex shapes. 
Oscillators can be used to generate a wide range of frequencies including the audio spectrum. These can be combined together to create more complex timbres (additive synthesis) and can be used to modulate other parameters such as another oscillator's frequency (frequency modulation) or amplitude (amplitude modulation). A digital oscillator may use a portion of sampled sound as a waveform.