<hardware> Cable that can be (legally) installed in the plenum (the space between a ceiling and the floor above it). Such cable is (usually) teflon-coated so that it will not give off toxic fumes when burned. In some parts of the world such cable is required by law.
Try this search on Wikipedia, OneLook, Google
Nearby terms: PL/C « PL Cornell « PLD « plenum cable » pleonasm » plesiochronous » Plesiochronous Digital Hierarchy