The Collaborative International Dictionary of English v.0.48:

Allegory \Al"le*go*ry\, n.; pl. Allegories. [L. allegoria, Gr.
   ?, description of one thing under the image of another; ?
   other + ? to speak in the assembly, harangue, ? place of
   assembly, fr. ? to assemble: cf. F. all['e]gorie.]
   1. A figurative sentence or discourse, in which the principal
      subject is described by another subject resembling it in
      its properties and circumstances. The real subject is thus
      kept out of view, and we are left to collect the
      intentions of the writer or speaker by the resemblance of
      the secondary to the primary subject.
      [1913 Webster]

   2. Anything which represents by suggestive resemblance; an
      emblem.
      [1913 Webster]

   3. (Paint. & Sculpt.) A figure representation which has a
      meaning beyond notion directly conveyed by the object
      painted or sculptured.
      [1913 Webster]

   Syn: Metaphor; fable.

   Usage: Allegory, Parable. "An allegory differs both from
          fable and parable, in that the properties of persons
          are fictitiously represented as attached to things, to
          which they are as it were transferred. . . . A figure
          of Peace and Victory crowning some historical
          personage is an allegory. "I am the Vine, ye are the
          branches" [--John xv. 1-6] is a spoken allegory. In
          the parable there is no transference of properties.
          The parable of the sower [--Matt. xiii. 3-23]
          represents all things as according to their proper
          nature. In the allegory quoted above the properties of
          the vine and the relation of the branches are
          transferred to the person of Christ and His apostles
          and disciples." --C. J. Smith.
          [1913 Webster]

   Note: An allegory is a prolonged metaphor. Bunyan's
         "Pilgrim's Progress" and Spenser's "Fa["e]rie Queene"
         are celebrated examples of the allegory.
         [1913 Webster]

WordNet (r) 3.0 (2006):

allegory
    n 1: a short moral story (often with animal characters) [syn:
         fable, parable, allegory, apologue]
    2: a visible symbol representing an abstract idea [syn:
       emblem, allegory]
    3: an expressive style that uses fictional characters and events
       to describe some subject by suggestive resemblances; an
       extended metaphor

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "allegory":
   Marchen, Western, Western story, Westerner, adventure story,
   allusion, analogy, apologue, arcane meaning, assumption, balancing,
   bedtime story, charactery, cipher, coloration, comparative anatomy,
   comparative degree, comparative grammar, comparative judgment,
   comparative linguistics, comparative literature,
   comparative method, compare, comparing, comparison, confrontation,
   confrontment, connotation, contrast, contrastiveness,
   conventional symbol, correlation, detective story, distinction,
   distinctiveness, emblem, fable, fabliau, fairy tale, fantasy,
   fiction, figuration, folk story, folktale, gest, ghost story, hint,
   horse opera, iconology, ideogram, implication, implied meaning,
   import, inference, innuendo, intimation, ironic suggestion, legend,
   likening, logogram, logotype, love knot, love story, matching,
   meaning, metaphor, metaphorical sense, mystery, mystery story,
   myth, mythology, mythos, nuance, nursery tale, occult meaning,
   opposing, opposition, overtone, parable, parallelism, pictogram,
   presumption, presupposition, proportion, relation, romance,
   science fiction, shocker, simile, similitude, space fiction,
   space opera, subsense, subsidiary sense, suggestion, supposition,
   suspense story, symbol, symbolic system, symbolism, symbolization,
   symbology, thriller, tinge, token, totem, totem pole, touch,
   trope of comparison, type, typification, undercurrent,
   undermeaning, undertone, weighing, whodunit, work of fiction



Easton's 1897 Bible Dictionary:

Allegory
   used only in Gal. 4:24, where the apostle refers to the history
   of Isaac the free-born, and Ishmael the slave-born, and makes
   use of it allegorically.

     Every parable is an allegory. Nathan (2 Sam. 12:1-4) addresses
   David in an allegorical narrative. In the eightieth Psalm there
   is a beautiful allegory: "Thou broughtest a vine out of Egypt,"
   etc. In Eccl. 12:2-6, there is a striking allegorical
   description of old age.