A system in which several things are happening at once.

There are several computer-related examples:

1. In a CPU the next instruction might be being decoded while the current one is being executed, that's a PipeLine.
2. In a CommandLine the second command might start processing the output of the first command before the first command has terminated, that's also a PipeLine.

CommandLine PipeLine hints

There is a nifty little program called "pipe viewer" (debian/ubuntu - install "pv") that can give you a progress meter based on how much of a file has been processed.

Eg, instead of grep pattern hugefile, you can pv hugefile | grep pattern. You can also insert it into the middle of a pipeline and give it options telling it the length of stdin, and if it can't work out the length it can give you statistics such as how fast the data is being processed. Neat.