The Collaborative International Dictionary of English v.0.48:

Intentionally \In*ten"tion*al*ly\, adv.
   In an intentional manner; with intention; by design; of
   purpose.
   [1913 Webster]

WordNet (r) 3.0 (2006):

intentionally
    adv 1: with intention; in an intentional manner; "he used that
           word intentionally"; "I did this by choice" [syn:
           intentionally, deliberately, designedly, on
           purpose, purposely, advisedly, by choice, by
           design] [ant: accidentally, by chance,
           circumstantially, unexpectedly, unintentionally]