The Collaborative International Dictionary of English v.0.48:

Deterrent \De*ter"rent\, a. [L. deterrens, p. pr. of deterrere.
   See Deter.]
   Serving to deter. "The deterrent principle." --E. Davis.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Deterrent \De*ter"rent\, n.
   That which deters or prevents.
   [1913 Webster]

WordNet (r) 3.0 (2006):

deterrent
    adj 1: tending to deter; "the deterrent effects of high prices"
    n 1: something immaterial that interferes with or delays action
         or progress [syn: hindrance, hinderance, deterrent,
         impediment, balk, baulk, check, handicap]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "deterrent":
   admonishing, admonitory, alarming, awing, bar, block, blockade,
   bottleneck, catch, cautionary, cautioning, check, chill, chilling,
   cold water, cordon, curtain, damp, damper, daunting, determent,
   deterring, difficulty, disconcerting, discouragement, discouraging,
   disheartening, disincentive, disinclining, dismaying, disquieting,
   dissuading, dissuasion, dissuasive, drawback, exemplary,
   expostulatory, fear-inspiring, fearful, fearsome, forbidding,
   forestalling, frightening, frightful, hang-up, hazard, hindrance,
   hitch, hurdle, impediment, intimidating, joker, monitorial,
   monitory, notificational, notifying, objection, obstacle,
   obstruction, obstructive, one small difficulty, overawing,
   preclusive, preventative, preventive, prohibitive, prophylactic,
   rub, scaring, scary, snag, startling, stumbling block,
   stumbling stone, warning, wet blanket