Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. The act of declaring or signifying; declaration;
      utterance; as, an expression of the public will.

            With this tone of philosophy were mingled
            expressions of sympathy.              --Prescott.

   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.

            The imitators of Shakespeare, fixing their attention
            on his wonderful power of expression, have directed
            their imitation to this.              --M. Arnold.

   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.''

Source : WordNet®

expression
     n 1: the expression on a person's face; "a sad expression"; "a
          look of triumph"; "an angry face" [syn: {look}, {aspect},
           {facial expression}, {face}]
     2: expression without words; "tears are an expression of
        grief"; "the pulse is a reflection of the heart's
        condition" [syn: {manifestation}, {reflection}, {reflexion}]
     3: the communication (in speech or writing) of your beliefs or
        opinions; "expressions of good will"; "he helped me find
        expression for my ideas"
     4: a word or phrase that particular people use in particular
        situations; "pardon the expression" [syn: {saying}, {locution}]
     5: the style of expressing yourself; "he suggested a better
        formulation"; "his manner of expression showed how much he
        cared" [syn: {formulation}]
     6: a group of symbols that make a mathematical statement [syn:
        {formula}]
     7: (genetics) the process of expressing a gene
     8: a group of words that form a constituent of a sentence and
        are considered as a single unit; "I concluded from his
        awkward constructions that he was a foreigner" [syn: {construction},
         {grammatical construction}] [ant: {misconstruction}]
     9: the act of forcing something out by squeezing or pressing;
        "the expression of milk from her breast"

Source : Free On-Line Dictionary of Computing

expression
     
         Any piece of program code in a {high-level
        language} which, when (if) its execution terminates, returns a
        value.  In most programming languages, expressions consist of
        constants, variables, operators, functions, and {parentheses}.
        The operators and functions may be built-in or user defined.
        Languages differ on how expressions of different {types} may
        be combined - with some combination of explicit {casts} and
        implicit {coercions}.
     
        The {syntax} of expressions generally follows conventional
        mathematical notation, though some languages such as {Lisp} or
        {Forth} have their own idiosyncratic syntax.
     
        (2001-05-14)