The Collaborative International Dictionary of English v.0.48:

Viscount \Vis"count`\, n. [OE. vicounte, OF. visconte, vescunte,
   F. vicomte, LL. vicecomes; L. vice (see Vice, a.) + comes a
   companion, LL., a count. See Count.]
   1. (O. Eng. Law) An officer who formerly supplied the place
      of the count, or earl; the sheriff of the county.
      [1913 Webster]

   2. A nobleman of the fourth rank, next in order below an earl
      and next above a baron; also, his degree or title of
      nobility. See Peer, n., 3. [Eng.] --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

viscount
    n 1: (in various countries) a son or younger brother or a count
    2: a British peer who ranks below an earl and above a baron

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "viscount":
   Brahman, archduke, aristocrat, armiger, baron, baronet, blue blood,
   count, daimio, duke, earl, esquire, gentleman, grand duke, grandee,
   hidalgo, lace-curtain, laird, landgrave, lord, lordling, magnate,
   magnifico, margrave, marquis, noble, nobleman, optimate, palsgrave,
   patrician, peer, seigneur, seignior, silk-stocking, squire, swell,
   thoroughbred, upper-cruster, waldgrave