What is the meaning of Flue pipe?

A pipe, especially an organ pipe, whose tone is produced by the impinging of a current of air upon an edge or lip, causing a wave motion in the air within; a mouth pipe; -- distinguished from reed pipe. Flue pipes are either open or closed (stopped at the distant end). The flute and flageolet are open pipes; a bottle acts as a closed pipe when one blows across the neck. The organ has both open and closed flue pipes, those of metal being usually round in section, and those of wood triangular or square.