The Collaborative International Dictionary of English v.0.48:

Flue pipe \Flue pipe\ (Music)
   A pipe, esp. 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.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

flue pipe
    n 1: organ pipe whose tone is produced by air passing across the
         sharp edge of a fissure or lip [syn: flue pipe, flue,
         labial pipe]