The Collaborative International Dictionary of English v.0.48:

Parenthesis \Pa*ren"the*sis\ (p[.a]*r[e^]n"th[-e]*s[i^]s), n.;
   pl. Parentheses. [NL., fr. Gr. pare`nqesis, fr.
   parentiqe`nai to put in beside, insert; para` beside + 'en in
   + tiqe`nai to put, place. See Para-, En-, 2, and
   Thesis.]
   [1913 Webster]
   1. A word, phrase, or sentence, by way of comment or
      explanation, inserted in, or attached to, a sentence which
      would be grammatically complete without it. It is usually
      inclosed within curved lines (see def. 2 below), or
      dashes. "Seldom mentioned without a derogatory
      parenthesis." --Sir T. Browne.
      [1913 Webster]

            Don't suffer every occasional thought to carry you
            away into a long parenthesis.         --Watts.
      [1913 Webster]

   2. (Print.) One of the curved lines () which inclose a
      parenthetic word or phrase.
      [1913 Webster]

   Note: Parenthesis, in technical grammar, is that part of a
         sentence which is inclosed within the recognized sign;
         but many phrases and sentences which are punctuated by
         commas are logically parenthetical. In def. 1, the
         phrase "by way of comment or explanation" is inserted
         for explanation, and the sentence would be
         grammatically complete without it. The present tendency
         is to avoid using the distinctive marks, except when
         confusion would arise from a less conspicuous
         separation.
         [1913 Webster]

WordNet (r) 3.0 (2006):

parenthesis
    n 1: either of two punctuation marks (or) used to enclose
         textual material
    2: a message that departs from the main subject [syn:
       digression, aside, excursus, divagation,
       parenthesis]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "parenthesis":
   anastrophe, aside, broken thread, brokenness, chiasmus,
   disconnectedness, disconnection, discontinuance, discontinuation,
   discontinuity, discontinuousness, discreteness, disjunction,
   episode, fitfulness, hypallage, hyperbaton, hysteron proteron,
   incoherence, incompleteness, infix, injection, insert, insertion,
   insinuation, intercalation, interjection, interlineation,
   interlocution, intermittence, interpolation, introduction,
   irregularity, metastasis, metathesis, non sequitur, noncontinuance,
   nonlinearity, nonseriality, nonuniformity, obiter dictum,
   palindrome, remark, side remark, synchysis, tmesis, tossing-in