Other forms: goodbyes
Goodbye is a parting word, something you say to other people when you're leaving. You might say goodbye to all your favorite teachers on your last day of high school.
A goodbye means that someone's departing: you say goodbye to your parents when you go off to college, and you also say goodbye to guests when they leave after a visit. The original goodbye, dating from the 1570s, was godbwye, which was a contraction of the farewell phrase "God be with ye!"