An Acronym for Coder / Decoder.

In communication systems, the term refers to components in the transmitter (contains a coder) and receiver (contains a decoder) which transform the data stream between the internal representation and a signal suitable for the physical media between the transmitter/receiver. Such CoDec pairs commonly perform exotic ErrorCorrection and insert synchronisation and timestamping features into the stream.

Along similar lines, on desktop computers, the term refers to a library (or a set thereof) which encapsulates access to a certain data format (commonly audio or video). The term is usually used in relation to MediaPlayers, who commonly use CoDecs as interchangable backends to support various media formats. For example, to view DivX files on your computer you will need a DivX CoDec for your MediaPlayer.