The Collaborative International Dictionary of English v.0.48:

Tophet \To"phet\, n. [Heb. t[=o]phet, literally, a place to be
   spit upon, an abominable place, fr. t[=u]ph to spit out.]
   A place lying east or southeast of Jerusalem, in the valley
   of Hinnom. [Written also Topheth.]
   [1913 Webster]

         And he defiled Topheth, which is in the valley of the
         children of Hinnom.                      --2 Kings
                                                  xxiii. 10.
   [1913 Webster]

   Note: It seems to have been at first part of the royal
         garden, but it was afterwards defiled and polluted by
         the sacrifices of Baal and the fires of Moloch, and
         resounded with the cries of burning infants. At a later
         period, its altars and high places were thrown down,
         and all the filth of the city poured into it, until it
         became the abhorrence of Jerusalem, and, in symbol, the
         place where are wailing and gnashing of teeth.
         [1913 Webster]

               The pleasant valley of Hinnom, Tophet thence
               And black Gehenna called, the type of hell.
                                                  --Milton.
         [1913 Webster]

Easton's 1897 Bible Dictionary:

Tophet
   =Topheth, from Heb. toph "a drum," because the cries of children
   here sacrificed by the priests of Moloch were drowned by the
   noise of such an instrument; or from taph or toph, meaning "to
   burn," and hence a place of burning, the name of a particular
   part in the valley of Hinnom. "Fire being the most destructive
   of all elements, is chosen by the sacred writers to symbolize
   the agency by which God punishes or destroys the wicked. We are
   not to assume from prophetical figures that material fire is the
   precise agent to be used. It was not the agency employed in the
   destruction of Sennacherib, mentioned in Isa. 30:33...Tophet
   properly begins where the Vale of Hinnom bends round to the
   east, having the cliffs of Zion on the north, and the Hill of
   Evil Counsel on the south. It terminates at Beer 'Ayub, where it
   joins the Valley of Jehoshaphat. The cliffs on the southern side
   especially abound in ancient tombs. Here the dead carcasses of
   beasts and every offal and abomination were cast, and left to be
   either devoured by that worm that never died or consumed by that
   fire that was never quenched." Thus Tophet came to represent the
   place of punishment. (See HINNOM.)


Hitchcock's Bible Names Dictionary (late 1800's):

Tophet, a drum; betraying