The Collaborative International Dictionary of English v.0.48:

Gehenna \Ge*hen"na\ (g[-e]*h[e^]n"n[.a]), prop. n. [L. Gehenna,
   Gr. Ge`enna, Heb. G[=e] Hinn[=o]m.] (Jewish Hist.)
   The valley of Hinnom, near Jerusalem, where some of the
   Israelites sacrificed their children to Moloch, which, on
   this account, was afterward regarded as a place of
   abomination, and made a receptacle for all the refuse of the
   city, perpetual fires being kept up in order to prevent
   pestilential effluvia. In the New Testament the name is
   transferred, by an easy metaphor, to Hell.
   [1913 Webster]

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

WordNet (r) 3.0 (2006):

Gehenna
    n 1: a place where the wicked are punished after death [syn:
         Gehenna, Tartarus]

Easton's 1897 Bible Dictionary:

Gehenna
   (originally Ge bene Hinnom; i.e., "the valley of the sons of
   Hinnom"), a deep, narrow glen to the south of Jerusalem, where
   the idolatrous Jews offered their children in sacrifice to
   Molech (2 Chr. 28:3; 33:6; Jer. 7:31; 19:2-6). This valley
   afterwards became the common receptacle for all the refuse of
   the city. Here the dead bodies of animals and of criminals, and
   all kinds of filth, were cast and consumed by fire kept always
   burning. It thus in process of time became the image of the
   place of everlasting destruction. In this sense it is used by
   our Lord in Matt. 5:22, 29, 30; 10:28; 18:9; 23:15, 33; Mark
   9:43, 45, 47; Luke 12:5. In these passages, and also in James
   3:6, the word is uniformly rendered "hell," the Revised Version
   placing "Gehenna" in the margin. (See HELL; HINNOM.)