An amplifier (or "amp") is an electronic device that increases the strength (power or voltage) of an electrical signal.

The amount of amplification, called gain, can be expressed in two ways:

1. As a multiplication factor: For example, a gain of x10 means that the signal voltage at the output is ten times greater than at the input.

2. In decibels (dB): For example, a gain of 20 dB corresponds to a 10-fold increase in signal voltage.

In the world of audio, there are several types of amplifiers, such as pre-amplifiers (which boost very weak signals, such as microphone signals, to line level) and power amplifiers (which provide the power needed to drive the loudspeakers).