Source : Webster's Revised Unabridged Dictionary (1913)

Parenthetically \Par`en*thet"ic*al*ly\, adv.
   In a parenthetical manner; by way of parenthesis; by
   parentheses.

Source : WordNet®

parenthetically
     adv : in a parenthetical manner; "he added parenthetically that he
           would not attend the wedding ceremony"