A channel is a model for interprocess communication and synchronization via message passing. A message may be sent over a channel, and another process or thread is able to receive messages sent over a channel it has a reference to, as a stream. Different implementations of channels may be buffered or not, and either synchronous or asynchronous.
Channels are fundamental to the process calculus approach to concurrency, and originated in communicating sequential processes (CSP), a formal model for concurrency, and has been used in many derived languages.