preamp

Strictly speaking, any electronic device that comes before the amplifier in a sound system.

In reality however it almost always refers to some kind of gain control. Whether it be in a mixer or a DI (Direct Input) box. The primary function of these is to raise the signal from mic level to line level. A good preamp will also considerably enhance the tone, giving the audio a warmer tone.

The preamp takes a signal from a source, such as a CD player, processes that signal, and sends it on to the amplifier. One example of signal processing is splitting it appropriately among two or more speakers, for stereo or surround sound.

In a basic home audio system, the preamp is often in the same box as the tuner and amplifier, the entirety of which is called a receiver. Higher-end preamps are often combined with a tuner, but left separate from the amp. The less stuff you have in one box, the less likely you are going to have components interfering with each other.