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.
      [1913 Webster]

   2. The act of declaring or signifying; declaration;
      utterance; as, an expression of the public will.
      [1913 Webster]

            With this tone of philosophy were mingled
            expressions of sympathy.              --Prescott.
      [1913 Webster]

   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.
      [1913 Webster]

            The imitators of Shakespeare, fixing their attention
            on his wonderful power of expression, have directed
            their imitation to this.              --M. Arnold.
      [1913 Webster]

   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.
      [1913 Webster]

            It still wore the majesty of expression so
            conspicuous in his portraits by the inimitable
            pencil of Titian.                     --Prescott.
      [1913 Webster]

   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.
      [1913 Webster]

   6. (Math.) The representation of any quantity or relation by
      appropriate characters or symbols, usually in a specific
      order.
      [1913 Webster +PJC]

   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,
      [PJC]

   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.
      [PJC]

   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.
         [PJC]

   Past expression, Beyond expression, beyond the power of
      description. "Beyond expression bright." --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

expression
    n 1: the feelings expressed on a person's face; "a sad
         expression"; "a look of triumph"; "an angry face" [syn:
         expression, 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:
       expression, manifestation, reflection, reflexion]
    3: the communication (in speech or writing) of your beliefs or
       opinions; "expressions of good will"; "he helped me find
       verbal expression for my ideas"; "the idea was immediate but
       the verbalism took hours" [syn: expression, verbal
       expression, verbalism]
    4: a word or phrase that particular people use in particular
       situations; "pardon the expression" [syn: saying,
       expression, locution]
    5: the style of expressing yourself; "he suggested a better
       formulation"; "his manner of expression showed how much he
       cared" [syn: formulation, expression]
    6: a group of symbols that make a mathematical statement [syn:
       formula, expression]
    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, expression]
       [ant: misconstruction]
    9: the act of forcing something out by squeezing or pressing;
       "the expression of milk from her breast"

Moby Thesaurus II by Grady Ward, 1.0:

292 Moby Thesaurus words for "expression":
   Christophany, Parthian shot, Satanophany, accent, adage, address,
   adjectival phrase, affirmation, air, airing, allegation, ana,
   analects, angelophany, announcement, answer, antonym, aphorism,
   apostrophe, apothegm, appearance, ardor, articulateness,
   articulation, aspect, assertion, asseveration, avatar, averment,
   avulsion, axiom, byword, cantando, cast, catchword,
   characterization, choice of words, clause, collected sayings,
   command of language, command of words, comment, communication,
   composition, concentration, construction, countenance, crack,
   current saying, cutting out, declaration, decoction, delivery,
   demilegato, demonstration, denomination, denotation, depth,
   deracination, designation, dialect, dictate, diction, dictum,
   differentiation, disclosure, disentanglement, dissemination,
   distich, distillation, drawing, drawing out, dredging, drilling,
   effective style, eloquence, eloquent tongue, embodiment, emotion,
   enucleation, enunciation, epigram, epiphany, eradication, evidence,
   evincement, evolvement, evulsion, excavation, excision,
   exclamation, execution, expressiveness, exsection, extirpation,
   extraction, extrication, face, facundity, feeling, felicitousness,
   felicity, fingering, formulation, free form, gesture,
   gift of expression, gift of gab, glibness, glissando, gnome,
   golden saying, grammar, graphicness, greeting, headed group, hint,
   homograph, homonym, homophone, identification, idiom, idiotism,
   incarnation, indication, indicativeness, infusion, intensity,
   interjection, intonation, issue, language, legato, lexeme,
   linguistic form, locution, logos, look, loudness, manifestation,
   manner of speaking, materialization, maxim, meaning,
   meaningfulness, mention, metonym, mezzo staccato, mien,
   minimum free form, mining, monosyllable, moral, mot, motto,
   music-making, naming, note, noun phrase, nuance, observation,
   oracle, oratory, paragraph, parlance, parlando, passion, pathos,
   peculiar expression, performance, period, phrasal idiom, phrase,
   phraseology, phrasing, pianism, picking out, pithy saying,
   pizzicato, pneumatophany, pointing, pointing out, pointing to,
   polysyllable, position, precept, prescript, pressing, pressing out,
   pronouncement, proof, proverb, proverbial saying, proverbs,
   publication, pulling, quarrying, question, reflection, remark,
   reminder, removal, rendering, rendition, repercussion,
   representation, revelation, rhetoric, ripping out, rubato, saw,
   say, saying, selection, sensitivity, sentence,
   sententious expression, set phrase, shading, show, showing, sign,
   signification, silver tongue, slickness, sloka, slur, smoothness,
   soaking, softness, specification, speech, spiccato, spirit,
   squeezing, squeezing out, staccato, standard phrase, statement,
   steeping, stock saying, style, subjoinder, suggestion, sutra,
   syllable, symbol, symptomaticness, synonym, syntactic structure,
   talk, teaching, term, text, theophany, thought, token, tone, touch,
   turn of expression, turn of phrase, unrooting, uprooting, usage,
   use of words, usus loquendi, utterance, vent, verb complex,
   verb phrase, verbalism, verbalization, verbiage, verbum, verse,
   visage, vividness, vocable, voice, voicing, way of speaking,
   wisdom, wisdom literature, wise saying, withdrawal, witticism,
   word, word-group, wordage, wording, words of wisdom,
   wresting out



The Free On-line Dictionary of Computing (30 December 2018):

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)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

EXPRESSION. The term or use of language employed to explain a thing.
     2. It is a general rule, that expressions shall be construed, when they
are capable of several significations, so as to give operation to the
agreement, act, or will, if it can be done; and an expression is always to
be understood in the sense most agreeable to the nature of the contract.
Vide Clause; Construction; Equivocal; Interpretation; Words.