The Collaborative International Dictionary of English v.0.48:

Adios \A`dios"\ ([aum]`dy[-o]s"), interj. [Sp., fr. L. ad to +
   deus god. Cf. Adieu.]
   Adieu; farewell; good-by; -- chiefly used among
   Spanish-speaking people; as, adios amigos (good-bye friends).
   [Webster 1913 Suppl.]

   Note: This word is often pronounced [.a]*d[=e]"[-o]s, but the
         Spanish accent, though weak, is on the final syllable.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

adios
    n 1: a farewell remark; "they said their good-byes" [syn:
         adieu, adios, arrivederci, auf wiedersehen, au
         revoir, bye, bye-bye, cheerio, good-by, goodby,
         good-bye, goodbye, good day, sayonara, so long]