The Collaborative International Dictionary of English v.0.48:

parenthetic \par`en*thet"ic\ (p[a^]r`[e^]n*th[e^]t"[i^]k),
parenthetical \par`en*thet"ic*al\
   (p[a^]r`[e^]n*th[e^]t"[i^]*kal), a. [Cf. Gr. pare`nqetos.]
   1. Of the nature of a parenthesis; pertaining to, or
      expressed in, or as if in, a parenthesis; as, a
      parenthetical clause; a parenthetic remark; a
      parenthetical style. [WordNet sense 1]
      [1913 Webster]

            A parenthetical observation of Moses himself.
                                                  --Hales.
      [1913 Webster]

   2. Using or containing parentheses.
      [1913 Webster]

   3. Added within parentheses to amplify or explain; as,
      parenthetical remarks. [WordNet sense 2]
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

parenthetical
    adj 1: qualifying or explaining; placed or as if placed in
           parentheses; "parenthetical remarks" [syn: parenthetic,
           parenthetical]
    n 1: an expression in parentheses; "his writing was full of
         parentheticals" [syn: parenthetical expression,
         parenthetical]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "parenthetical":
   accidental, adrift, beside the mark, beside the point,
   beside the question, broken, broken off, by-the-way, casual,
   chopped-off, choppy, decousu, disconnected, discontinued,
   discontinuous, discrete, disjunctive, episodic, extraneous,
   extrinsic, fitful, herky-jerky, immaterial, impertinent,
   inadmissible, inapplicable, inapposite, inappropriate, incidental,
   incoherent, inconsequent, intercalary, interjectional,
   intermittent, interpolative, interrupted, irregular, irrelative,
   irrelevant, jagged, jerky, nihil ad rem, noncontinuous,
   nonessential, nonlinear, nonsequential, nonserial, nonuniform,
   not at issue, occasional, off the subject, out-of-the-way,
   parenthetic, patchy, scrappy, snatchy, spasmodic, spotty,
   suspended, unconnected, unessential, unjoined, unsuccessive