The opposite of an amplifier; another way of putting it is that an attenuator is an "amplifier" with a gain of less than 1. Most voltage controlled amplifiers used in synths are actually voltage controlled atteuators; the maximum output level is at a gain of 1, and all other values and control voltages produce a gain of less than 1.
A non-voltage controlled attenuator can be built as a simple passive circuit. These are sometimes used in synth to prevent a signal from overdriving a circuit input. They can also be combined to produce a passive mixer.