The Collaborative International Dictionary of English v.0.48:

Expression \Ex*pres"sion\ ([e^]ks*pr[e^]sh"[u^]n), n. [L.
   expressio: cf. F. expression.]
   1. The act of expressing; the act of forcing out by pressure;
      as, the expression of juices or oils; also, of extorting
      or eliciting; as, a forcible expression of truth.
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   2. The act of declaring or signifying; declaration;
      utterance; as, an expression of the public will.
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            With this tone of philosophy were mingled
            expressions of sympathy.              --Prescott.
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   3. Lively or vivid representation of meaning, sentiment, or
      feeling, etc.; significant and impressive indication,
      whether by language, appearance, or gesture; that manner
      or style which gives life and suggestive force to ideas
      and sentiments; as, he reads with expression; her
      performance on the piano has expression.
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            The imitators of Shakespeare, fixing their attention
            on his wonderful power of expression, have directed
            their imitation to this.              --M. Arnold.
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   4. That which is expressed by a countenance, a posture, a
      work of art, etc.; look, as indicative of thought or
      feeling. "The expression of an eye." --Tennyson.
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            It still wore the majesty of expression so
            conspicuous in his portraits by the inimitable
            pencil of Titian.                     --Prescott.
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   5. A form of words in which an idea or sentiment is conveyed;
      a mode of speech; a phrase; as, a common expression; an
      odd expression.
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   6. (Math.) The representation of any quantity or relation by
      appropriate characters or symbols, usually in a specific
      order.
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   7. (Genetics) the production of products by a gene that cause
      the appearance of the corresponding protein or phenotype;
      -- of a gene or of an organism with a specific gene; as,
      the expression the beta-galactosidase positive phenotype,
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   8. (Computers) a combination of characters linked by
      operators, occurring as part of the code of a computer
      program, which must be evaluated according to the rules of
      the computer language in order to produce a resulting
      value.
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   Note: In most programming languages, (a + b) is an expression
         indicating simple arithmetic addition, if the variables
         a and b are real numbers. Many other types of operation
         may be used in programs, and each set of symbols
         indicating an operation is an expression in that
         program.
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   Past expression, Beyond expression, beyond the power of
      description. "Beyond expression bright." --Milton.
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