The term codec is a blending of the words "compressor-decompressor" or "coder-decoder". A codec is a device or computer program that encodes or decodes a digital data stream or signal. It converts analog audio or video to digital format, and then computer digital sound or video back to analog audio or video.

As there are many multimedia formats available, VoIP and other systems often use a variety of codecs, ranging from those that handle highly compressed voice-quality audio for limited bandwidth applications to codecs for high-fidelity stereo and high-definition video.