The Collaborative International Dictionary of English v.0.48:

Concept \Con"cept\, n. [L. conceptus (cf. neut. conceptum
   fetus), p. p. of concipere to conceive: cf. F. concept. See
   Conceit.]
   An abstract general conception; a notion; a universal.
   [1913 Webster]

         The words conception, concept, notion, should be
         limited to the thought of what can not be represented
         in the imagination; as, the thought suggested by a
         general term.                            --Sir W.
                                                  Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

concept
    n 1: an abstract or general idea inferred or derived from
         specific instances [syn: concept, conception,
         construct] [ant: misconception]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "concept":
   Vorstellung, apprehension, assumption, attitude,
   climate of opinion, common belief, community sentiment, conceit,
   conception, conceptualization, conclusion, consensus gentium,
   consideration, eidetic image, envisaging, envisioning, estimate,
   estimation, ethos, eye, fancy, feeling, general belief, idea,
   image, imagery, imagery study, imaging, imagism, imagistic poetry,
   imago, impression, intellection, intellectual object, judgment,
   lifelike image, lights, memory-trace, mental image,
   mental impression, mental picture, mental representation, mind,
   mystique, notion, objectification, observation, opinion,
   perception, personal judgment, picture, picturing, poetic imagery,
   point of view, popular belief, position, posture, presumption,
   prevailing belief, public belief, public opinion, reaction, recept,
   reflection, representation, sentiment, sight, stance, supposition,
   theory, thinking, thought, view, vision, visual image,
   visualization, way of thinking, word-painting