The Collaborative International Dictionary of English v.0.48:

consultant \consultant\ n.
   an expert who gives advice.

   Syn: adviser, advisor.
        [WordNet 1.5]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "consultant":
   Admirable Crichton, Dutch uncle, Polonius, adept, admonisher,
   adviser, advisor, artisan, artist, attache, authority,
   backseat driver, buttinsky, confidant, connaisseur, connoisseur,
   cordon bleu, counsel, counselor, crack shot, craftsman, dead shot,
   diplomat, diplomatist, doctor, elder statesman, experienced hand,
   expert, expert consultant, graduate, guide, handy man, instructor,
   journeyman, kibitzer, marksman, meddler, mentor, monitor, nestor,
   no slouch, orienter, physician, politician, preceptist, pro,
   professional, professor, proficient, savant, shark, sharp,
   specialist, statesman, teacher, technical adviser, technician