good·bye or good-bye also good-by 
(gd-bī)
(gd-bī)interj.
Used to express an acknowledgment of parting.
n. pl. good·byes or good-byes also good-bys
1. An acknowledgment at parting, especially by saying "goodbye."
2. An act of parting or leave-taking: many sad goodbyes.
[Alteration (influenced by good day) of God be with you.]