The Collaborative International Dictionary of English v.0.48:

Notion \No"tion\, [L. notio, fr. noscere to know: cf. F. notion.
   See Know.]
   1. Mental apprehension of whatever may be known or imagined;
      an idea; a conception; more properly, a general or
      universal conception, as distinguishable or definable by
      marks or notae.
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            What hath been generally agreed on, I content myself
            to assume under the notion of principles. --Sir I.
                                                  Newton.
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            Few agree in their notions about these words.
                                                  --Cheyne.
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            That notion of hunger, cold, sound, color, thought,
            wish, or fear which is in the mind, is called the
            "idea" of hunger, cold, etc.          --I. Watts.
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            Notion, again, signifies either the act of
            apprehending, signalizing, that is, the remarking or
            taking note of, the various notes, marks, or
            characters of an object which its qualities afford,
            or the result of that act.            --Sir W.
                                                  Hamilton.
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   2. A sentiment; an opinion.
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            The extravagant notion they entertain of themselves.
                                                  --Addison.
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            A perverse will easily collects together a system of
            notions to justify itself in its obliquity. --J. H.
                                                  Newman.
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   3. Sense; mind. [Obs.] --Shak.
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   4. An invention; an ingenious device; a knickknack; as,
      Yankee notions. [Colloq.]
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   5. Inclination; intention; disposition; as, I have a notion
      to do it. [Colloq.]
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   6. Miscellaneous small objects; sundries; -- usually
      referring to articles displayed together for sale.
      [PJC]

WordNet (r) 3.0 (2006):

notion
    n 1: a vague idea in which some confidence is placed; "his
         impression of her was favorable"; "what are your feelings
         about the crisis?"; "it strengthened my belief in his
         sincerity"; "I had a feeling that she was lying" [syn:
         impression, feeling, belief, notion, opinion]
    2: a general inclusive concept
    3: an odd or fanciful or capricious idea; "the theatrical notion
       of disguise is associated with disaster in his stories"; "he
       had a whimsy about flying to the moon"; "whimsy can be
       humorous to someone with time to enjoy it" [syn: notion,
       whim, whimsy, whimsey]
    4: (usually plural) small personal articles or clothing or
       sewing items; "buttons and needles are notions"

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "notion":
   apprehension, assumption, attitude, automatic response,
   bare suggestion, bee, blind impulse, boutade, brain wave,
   brainstorm, capriccio, caprice, climate of opinion, clue,
   common belief, community sentiment, conceit, concept, conception,
   conclusion, consensus gentium, consideration, crank, craze,
   crazy idea, crotchet, cue, drive, estimate, estimation, ethos, eye,
   fad, fancy, fantastic notion, fantasy, feeling, flash,
   fleeting impulse, flimflam, fool notion, freak,
   freakish inspiration, general belief, general idea, gut response,
   half an idea, harebrained idea, hazy idea, hint, humor, idea,
   image, imago, impression, impulse, inclination, indication,
   inkling, inspiration, instinct, intellection, intellectual object,
   intimation, involuntary impulse, judgment, kink, lights, maggot,
   megrim, memory-trace, mental image, mental impression,
   mental picture, mere notion, mind, mystique, natural impulse,
   observation, opinion, passing fancy, perception, personal judgment,
   point of view, popular belief, position, posture, presumption,
   prevailing belief, public belief, public opinion, quick hunch,
   quirk, reaction, recept, reflection, reflex, representation,
   sentiment, sight, sneaking suspicion, stance, sudden thought,
   suggestion, supposition, suspicion, telltale, theory, thinking,
   thought, toy, urge, vagary, vague idea, view, way of thinking,
   whim, whim-wham, whimsy, wind