Channel
A channel is a block of hardware (effectively a co-processor) which performs I/O operations, off-loading that work from the main CPU; they usually have direct access to main memory.
They are most commonly found on mainframe systems; on smaller machines, mechanisms such as DMA from device controllers do a lot of what a channel does.
The details vary considerably from manufacturer to manufacturer: for instance, the Peripheral Processing Units of the CDC 6600 are effectively channels, but they are essentially complete minicomputers (although they are implemented with shared hardware). The channels of IBM mainframes are programmable, but are much more limited in functionality; they are finite state machines, with conditional branching capability.
IBM channels come in two varieties: a 'multiplexer channel' handles a group of slow-speed devices, and interleaves operations to each of them, with each transfer handling single small data item (e.g. a byte); a 'selector channel' handles high-speed devices such as disks and magnetic tape drives, and transfers an entire block at a time.
Channels were apparently first introduced with the IBM 709, and other manufacturers copied the idea.
