The Collaborative International Dictionary of English v.0.48:

Impression \Im*pres"sion\, n. [F. impression, L. impressio.]
   1. The act of impressing, or the state of being impressed;
      the communication of a stamp, mold, style, or character,
      by external force or by influence.
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   2. That which is impressed; stamp; mark; indentation;
      sensible result of an influence exerted from without.
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            The stamp and clear impression of good sense.
                                                  --Cowper.
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            To shelter us from impressions of weather, we must
            spin, we must weave, we must build.   --Barrow.
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   3. That which impresses, or exercises an effect, action, or
      agency; appearance; phenomenon. [Obs.]
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            Portentous blaze of comets and impressions in the
            air.                                  --Milton.
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            A fiery impression falling from out of Heaven.
                                                  --Holland.
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   4. Influence or effect on the senses or the intellect hence,
      interest, concern. --Reid.
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            His words impression left.            --Milton.
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            Such terrible impression made the dream. --Shak.
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            I have a father's dear impression,
            And wish, before I fall into my grave,
            That I might see her married.         --Ford.
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   5. An indistinct notion, remembrance, or belief.
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   6. Impressiveness; emphasis of delivery.
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            Which must be read with an impression. --Milton.
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   7. (Print.) The pressure of the type on the paper, or the
      result of such pressure, as regards its appearance; as, a
      heavy impression; a clear, or a poor, impression; also, a
      single copy as the result of printing, or the whole
      edition printed at a given time; as, a copy from the fifth
      impression.
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            Ten impressions which his books have had. --Dryden.
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   8. In painting, the first coat of color, as the priming in
      house painting and the like. [R.]
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   9. (Engraving) A print on paper from a wood block, metal
      plate, or the like.
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   Proof impression, one of the early impressions taken from
      an engraving, before the plate or block is worn.
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WordNet (r) 3.0 (2006):

impression
    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: an outward appearance; "he made a good impression"; "I wanted
       to create an impression of success"; "she retained that bold
       effect in her reproductions of the original painting" [syn:
       impression, effect]
    3: a clear and telling mental image; "he described his mental
       picture of his assailant"; "he had no clear picture of
       himself or his world"; "the events left a permanent
       impression in his mind" [syn: mental picture, picture,
       impression]
    4: a concavity in a surface produced by pressing; "he left the
       impression of his fingers in the soft mud" [syn:
       depression, impression, imprint]
    5: a symbol that is the result of printing or engraving; "he put
       his stamp on the envelope" [syn: stamp, impression]
    6: all the copies of a work printed at one time; "they ran off
       an initial printing of 2000 copies" [syn: impression,
       printing]
    7: (dentistry) an imprint of the teeth and gums in wax or
       plaster; "the dentist took an impression for use in preparing
       an inlay"
    8: an impressionistic portrayal of a person; "he did a funny
       impression of a politician"
    9: the act of pressing one thing on or into the surface of
       another; "he watched the impression of the seal on the hot
       wax"