Tremolo

A regular and repetitive variation in amplitude for the duration of a single note; this is the most common meaning. Tremolo is amplitude (level) modulation and not to be confused with Vibrato which is frequency (or pitch) modulation.

Electronic instruments
A signal is passed through a VCA (voltage controlled amplifier) the output level of which is controlled by a low frequency oscillator (LFO) which is producing a rising and falling voltage. As the voltage increases, so the output level increases and vice versa. The LFO runs at around 5 cycles per second to produce the characteristic tremolo effect. Usually the rate/speed and intensity of the effect are variable.