WordNet (r) 3.0 (2006):

unidirectional
    adj 1: operating or moving or allowing movement in one direction
           only; "a unidirectional flow"; "a unidirectional
           antenna"; "a unidirectional approach to a problem" [ant:
           bidirectional]

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "unidirectional":
   direct, immediate, irreversible, monadal, monadic, one-piece,
   one-sided, one-way, straight, straightaway, straightforward,
   straightway, unbroken, undeviating, uniangulate, unibivalent,
   unibranchiate, unicameral, unicellular, unicuspid, unidentate,
   unidigitate, unidimensional, uniflorous, unifoliate, unifoliolate,
   unigenital, uniglobular, unilateral, unilinear, uniliteral,
   unilobed, unilobular, unimodular, unimolecular, uninterrupted,
   uniocular, unipart, unipartite, unipolar, univalent, univocal,
   unswerving