1.nounA conduit made of pipes used to convey water, gas or petroleum etc.
"An oil pipeline has been opened from the Caspian Sea."
2.nounA channel (either physical or logical) by which information is transmitted sequentially (that is, the first information in is the first information out).
"3D images are rendered using the graphics pipeline."
3.nounA system or process through which something is conducted.
"A new version of the software is in the pipeline, but has not been rolled out."
4.nounThe inside of a wave that a surfer is riding, when the wave has started closing over it.