'Dungeon' definitions:

Definition of 'dungeon'

noun

The main tower within the walls of a medieval castle or fortress [syn: keep, donjon, dungeon]

noun

A dark cell (usually underground) where prisoners can be confined

From: GCIDE

From: GCIDE

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]

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).