A piece of electronics (or code) that produces a continuous waveform at a stable frequency, such as a sine wave, a sawtooth or a rectable.
LFO (Low Frequency Oscillator)
An oscillator that runs a low frequencies, often way outside the audible range and is, instead of making sound directly, used to modulate parameters.
Envelope
A signal genereator that, depending on events (usually: A keyboard key pressed down and then released again) generates a smooth curve to shape a sound in a more natural way than just switching in on and off. The ADSR (Attack, Decay, Sustain, Release) envelope is the most common one as it is a good compromise between control and ease of use