The Collaborative International Dictionary of English v.0.48:

Symmetrical \Sym*met"ric*al\, a. [Cf. F. sym['e]trique. See
   Symmetry.]
   1. Involving or exhibiting symmetry; proportional in parts;
      having its parts in due proportion as to dimensions; as, a
      symmetrical body or building.
      [1913 Webster]

   2. (Biol.) Having the organs or parts of one side
      corresponding with those of the other; having the parts in
      two or more series of organs the same in number;
      exhibiting a symmetry. See Symmetry, 2.
      [1913 Webster]

   3. (Bot.)
      (a) Having an equal number of parts in the successive
          circles of floral organs; -- said of flowers.
      (b) Having a likeness in the form and size of floral
          organs of the same kind; regular.
          [1913 Webster]

   4. (Math.) Having a common measure; commensurable.
      (b) Having corresponding parts or relations.
          [1913 Webster]

   Note: A curve or a plane figure is symmetrical with respect
         to a given line, and a line, surface, or solid with
         respect to a plane, when for each point on one side of
         the line or plane there is a corresponding point on the
         other side, so situated that the line joining the two
         corresponding points is perpendicular to the line or
         plane and is bisected by it. Two solids are symmetrical
         when they are so situated with respect to an
         intervening plane that the several points of their
         surfaces thus correspond to each other in position and
         distance. In analysis, an expression is symmetrical
         with respect to several letters when any two of them
         may change places without affecting the expression; as,
         the expression a^2b + ab^2 + a^2c + ac^2 +
         b^2c + bc^2, is symmetrical with respect to the
         letters a, b, c.
         [1913 Webster] -- Sym*met"ric*al*ly, adv. --
         Sym*met"ric*al*ness, n.
         [1913 Webster]

WordNet (r) 3.0 (2006):

symmetrical
    adj 1: having similarity in size, shape, and relative position
           of corresponding parts [syn: symmetrical, symmetric]
           [ant: asymmetric, asymmetrical]
    2: exhibiting equivalence or correspondence among constituents
       of an entity or between different entities [syn:
       harmonious, proportionate, symmetrical]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "symmetrical":
   apoise, arranged, balanced, businesslike, coequal, commensurable,
   commensurate, concinnate, concinnous, congruent, congruous,
   coordinate, equal, equibalanced, equilateral, equiponderant,
   equiponderous, euphonic, euphonical, euphonious, eurythmic, even,
   finished, flowing, fluent, formal, habitual, harmonious, in hand,
   in proportion, measured, methodical, normal, ordered, orderly,
   poised, proportional, proportionate, proportioned, regular,
   routine, smooth, smooth-sounding, steady, sweet, symmetric,
   systematic, tripping, uniform, usual, well-balanced, well-ordered,
   well-proportioned, well-regulated, well-set, well-set-up