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.

In low-end sound equipment, the preamp is often in the same box as the tuner, receiver, and amplifier. Higher-end preamps are often combined with a tuner and receiver, 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.
If all you are doing is listening to music on a two-channel system, technically the only thing you need before the amplifier is a preamp. Unless you have a dedicated two-channel system, however, you might as well get a tuner/receiver/preamp, which is usually just called a receiver.