A Covert Channel is a covert communication channel through which information is transmitted. Such a channel is parasitic in nature and uses third party transmission media or information content to deliver its messages.
A covert channel is not necessarily a transmission channel. The parasitic information can be transmitted through a physical channel, but it can also be mixed with another information content as in the case of steganography. Another example is watermarking, which uses the blanking intervals of a television picture, or the transmission of information in pauses in speech.