A pipe is a man-made channel, closed on all sides, generally round in cross section, for transporting or guiding a fluid substance.
"Pipe" may also be used to refer to certain musical instruments such as the flute, the recorder, the pan pipe and the bagpipe (almost always referred to in the plural form "pipes"). Such instruments are characterized as being reed instruments shaped like a pipe or collection of pipes. "Piping" as a verb means playing one of these instruments.
There is also a geological "pipe" which it is up to someone else to describe.