The Collaborative International Dictionary of English v.0.48:

Similitude \Si*mil"i*tude\, n. [F. similitude, L. similitudo,
   from similis similar. See Similar.]
   1. The quality or state of being similar or like;
      resemblance; likeness; similarity; as, similitude of
      substance. --Chaucer.
      [1913 Webster]

            Let us make now man in our image, man
            In our similitude.                    --Milton.
      [1913 Webster]

            If fate some future bard shall join
            In sad similitude of griefs to mine.  --Pope.
      [1913 Webster]

   2. The act of likening, or that which likens, one thing to
      another; fanciful or imaginative comparison; a simile.
      [1913 Webster]

            Tasso, in his similitudes, never departed from the
            woods; that is, all his comparisons were taken from
            the country.                          --Dryden.
      [1913 Webster]

   3. That which is like or similar; a representation,
      semblance, or copy; a facsimile.
      [1913 Webster]

            Man should wed his similitude.        --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

similitude
    n 1: similarity in appearance or character or nature between
         persons or things; "man created God in his own likeness"
         [syn: likeness, alikeness, similitude] [ant:
         dissimilitude, unlikeness]
    2: a duplicate copy [syn: counterpart, similitude, twin]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "similitude":
   accordance, affinity, agreement, alikeness, allegory, alliance,
   ally, alter ego, analogon, analogue, analogy, aping, approach,
   approximation, assimilation, associate, balancing, brother,
   certified copy, close copy, close match, closeness, cognate,
   community, companion, comparability, comparative anatomy,
   comparative degree, comparative grammar, comparative judgment,
   comparative linguistics, comparative literature,
   comparative method, compare, comparing, comparison, complement,
   conformity, confrontation, confrontment, congenator, congener,
   contrast, contrastiveness, coordinate, copy, copying, correlate,
   correlation, correlative, correspondence, correspondent,
   counterfeit, counterpart, dead ringer, distinction,
   distinctiveness, double, duplicate, ectype, effigy, equivalent,
   exact likeness, fair copy, faithful copy, fake, fellow, forgery,
   icon, identity, idol, image, imitation, kindred spirit, like,
   likeness, likening, living image, living picture, match, matching,
   mate, metaphor, mimicking, miniature, mirroring, model,
   near duplicate, nearness, obverse, opposing, opposition, parallel,
   parallelism, parity, pasticcio, pastiche, pendant, phony,
   photograph, picture, portrait, proportion, reciprocal, reflection,
   relation, replica, representation, resemblance, rubbing, sameness,
   second self, semblance, shadow, similarity, simile, simulacrum,
   simulation, sister, soul mate, spit and image, spitting image,
   such, suchlike, tally, the like of, the likes of, trace, tracing,
   trope of comparison, twin, very image, very picture, weighing