Source : Free On-Line Dictionary of Computing

plenum cable
     
         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.
     
        (1996-08-25)