The Collaborative International Dictionary of English v.0.48:

dungeon \dun"geon\ (d[u^]n"j[u^]n), n. [OE. donjoun highest
   tower of a castle, tower, prison, F. donjon tower or platform
   in the midst of a castle, turret, or closet on the top of a
   house, a keep of a castle, LL. domnio, the same word as LL.
   dominus lord. See Dame, Don, and cf. Dominion,
   Domain, Demesne, Danger, Donjon.]
   A close, dark prison, commonly, under ground, as if the lower
   apartments of the donjon or keep of a castle, these being
   used as prisons.
   [1913 Webster]

         Down with him even into the deep dungeon. -- Tyndale.
   [1913 Webster]

         Year after year he lay patiently in a dungeon. --
                                                  Macaulay.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Dungeon \Dun"geon\, v. t.
   To shut up in a dungeon. --Bp. Hall.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dungeon
    n 1: the main tower within the walls of a medieval castle or
         fortress [syn: keep, donjon, dungeon]
    2: a dark cell (usually underground) where prisoners can be
       confined

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "dungeon":
   POW camp, bastille, black hole, borstal, borstal institution,
   bridewell, brig, cell, concentration camp, condemned cell,
   death cell, death house, death row, detention camp, donjon,
   federal prison, forced-labor camp, gaol, guardhouse,
   house of correction, house of detention, industrial school,
   internment camp, jail, jailhouse, keep, labor camp, lockup,
   maximum-security prison, minimum-security prison, oubliette, pen,
   penal colony, penal institution, penal settlement, penitentiary,
   prison, prison camp, prisonhouse, reform school, reformatory,
   sponging house, state prison, stockade, stronghold, the hole,
   tollbooth, training school, vault



The Free On-line Dictionary of Computing (30 December 2018):

Zork
Dungeon

    /zork/ The second of the great early experiments in
   computer fantasy gaming; see ADVENT.  Zork was originally
   written on MIT-DM during the late 1970s, later distributed
   with BSD Unix as a patched, sourceless RT-11 Fortran
   binary (see retrocomputing) and commercialised as "The Zork
   Trilogy" by Infocom.  The Fortran source was later rewritten
   for portability and released to Usenet under the name
   "Dungeon".

   Both Fortran "Dungeon" and translated C versions are
   available from many FTP archives.

   [Jargon File]

   (1998-09-21)


Easton's 1897 Bible Dictionary:

Dungeon
   different from the ordinary prison in being more severe as a
   place of punishment. Like the Roman inner prison (Acts 16:24),
   it consisted of a deep cell or cistern (Jer. 38:6). To be shut
   up in, a punishment common in Egypt (Gen. 39:20; 40:3; 41:10;
   42:19). It is not mentioned, however, in the law of Moses as a
   mode of punishment. Under the later kings imprisonment was
   frequently used as a punishment (2 Chron. 16:10; Jer. 20:2;
   32:2; 33:1; 37:15), and it was customary after the Exile (Matt.
   11:2; Luke 3:20; Acts 5:18, 21; Matt. 18:30).


Bouvier's Law Dictionary, Revised 6th Ed (1856):

DUNGEON. A cell under ground; a place in a prison built under ground, dark,
or but indifferently lighted. In the prisons of the United States, there are
few or no dungeons.