The Collaborative International Dictionary of English v.0.48:

Unicellular \U`ni*cel"lu*lar\, a. [Uni- + cellular.]
   Having, or consisting of, but a single cell; as, a
   unicellular organism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unicellular
    adj 1: having or consisting of a single cell

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "unicellular":
   cellular, cellulous, corpuscular, monadal, monadic, multicellular,
   one-piece, one-sided, uniangulate, unibivalent, unibranchiate,
   unicameral, unicuspid, unidentate, unidigitate, unidimensional,
   unidirectional, uniflorous, unifoliate, unifoliolate, unigenital,
   uniglobular, unilateral, unilinear, uniliteral, unilobed,
   unilobular, unimodular, unimolecular, uniocular, unipart,
   unipartite, unipolar, univalent, univocal