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]