In UNIX file systems, a special type of file that acts as a FIFO.
Basically, it's a file that refers not to data that resides on a disk, but rather to an input/output channel opened by another program.
Any process may open a named pipe for writing, and the execution of the process will halt until some program will open it for reading - or vice versa.
Named pipes are a rather old invention, and part of POSIX standard. In POSIX systems, the C library defines mkfifo() command for creating named pipes.