What term describes a circuit designed to increase the amplitude of a signal?

An amplifier is an electronic circuit specifically designed to increase the amplitude (which can mean power, voltage, or current) of an electrical signal. It takes a weak input signal and produces a proportionately stronger output signal. The other options describe circuits with entirely different primary functions. An oscillator generates a continuous signal from scratch, a modulator impresses information onto a carrier wave, and a multiplier is used to multiply the frequency of a signal, not its amplitude.