The Collaborative International Dictionary of English v.0.48:

Idea \I*de"a\, n.; pl. Ideas. [L. idea, Gr. ?, fr. ? to see;
   akin to E. wit: cf. F. id['e]e. See Wit.]
   1. The transcript, image, or picture of a visible object,
      that is formed by the mind; also, a similar image of any
      object whatever, whether sensible or spiritual.
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            Her sweet idea wandered through his thoughts.
                                                  --Fairfax.
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            Being the right idea of your father
            Both in your form and nobleness of mind. --Shak.
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            This representation or likeness of the object being
            transmitted from thence [the senses] to the
            imagination, and lodged there for the view and
            observation of the pure intellect, is aptly and
            properly called its idea.             --P. Browne.
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   2. A general notion, or a conception formed by
      generalization.
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            Alice had not the slightest idea what latitude was.
                                                  --L. Caroll.
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   3. Hence: Any object apprehended, conceived, or thought of,
      by the mind; a notion, conception, or thought; the real
      object that is conceived or thought of.
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            Whatsoever the mind perceives in itself, or as the
            immediate object of perception, thought, or
            undersanding, that I call idea.       --Locke.
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   4. A belief, option, or doctrine; a characteristic or
      controlling principle; as, an essential idea; the idea of
      development.
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            That fellow seems to me to possess but one idea, and
            that is a wrong one.                  --Johnson.
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            What is now "idea" for us? How infinite the fall of
            this word, since the time where Milton sang of the
            Creator contemplating his newly-created world,
            "how it showed . . .
            Answering his great idea,"
            to its present use, when this person "has an idea
            that the train has started," and the other "had no
            idea that the dinner would be so bad!" --Trench.
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   5. A plan or purpose of action; intention; design.
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            I shortly afterwards set off for that capital, with
            an idea of undertaking while there the translation
            of the work.                          --W. Irving.
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   6. A rational conception; the complete conception of an
      object when thought of in all its essential elements or
      constituents; the necessary metaphysical or constituent
      attributes and relations, when conceived in the abstract.
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   7. A fiction object or picture created by the imagination;
      the same when proposed as a pattern to be copied, or a
      standard to be reached; one of the archetypes or patterns
      of created things, conceived by the Platonists to have
      excited objectively from eternity in the mind of the
      Deity.
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            Thence to behold this new-created world,
            The addition of his empire, how it showed
            In prospect from his throne, how good, how fair,
            Answering his great idea.             --Milton.
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   Note: "In England, Locke may be said to have been the first
         who naturalized the term in its Cartesian universality.
         When, in common language, employed by Milton and
         Dryden, after Descartes, as before him by Sidney,
         Spenser, Shakespeare, Hooker, etc., the meaning is
         Platonic." --Sir W. Hamilton.
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   Abstract idea, Association of ideas, etc. See under
      Abstract, Association, etc.

   Syn: Notion; conception; thought; sentiment; fancy; image;
        perception; impression; opinion; belief; observation;
        judgment; consideration; view; design; intention;
        purpose; plan; model; pattern.

   Usage: There is scarcely any other word which is subjected to
          such abusive treatment as is the word idea, in the
          very general and indiscriminative way in which it is
          employed, as it is used variously to signify almost
          any act, state, or content of thought.
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