The Collaborative International Dictionary of English v.0.48:

Good-by \Good`-by"\, Good-bye \Good`-bye"\, n. or interj. [A
   contraction of God be with ye (God be w[i^] ye, God bw' ye,
   God bwye).]
   Farewell; a form of address used at parting. See the last
   Note under By, prep. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

goodbye
    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]