In Java 1.4+, Channel has a more specific meaning. It refers to anything you can do nio on, e.g. a file, a socket, a datagram, a pipe, a memory mapped file, a buffer… It is the root interface of a family of interfaces implemented by the new classes: DatagramChannel (which lets you work at the UDP datagram level), FileChannel (which lets you do locks, and memory mapping), and SocketChannel (which lets you shutdown input or output temporarily, and lets you monitor the progress of setting up the connection).

In image processing, channel refers to the the way you decompose an image pixel into four parts, the red, green, blue and alpha. Typically a pixel’s colour is represented by four numbers 0..255. The higher the number the brighter. The alpha channel measures transparency/opacity. 0 is 100% transparent. 255 is 100% opaque. 128 is 50% opaque, allowing what is below to partly show through. If you were talking only about the red component of an image, you would talk of the the red channel for the image.