Goodbye is defined as a way to express that you are leaving.(interjection)
An example of goodbye is what you'd say to the hosts of a party you're leaving.
See goodbye in Webster's New World College Dictionary
See goodbye in American Heritage Dictionary 4
or good-bye also good-byinterjection
Origin: Alteration (influenced by good day)
Origin: of God be with you. Word History: No doubt more than one reader has wondered exactly how goodbye is derived from the phrase “God be with you.” To understand this, it is helpful to see earlier forms of the expression, such as God be wy you, god b'w'y, godbwye, god buy' ye, and good-b'wy. The first word of the expression is now good and not God, for good replaced God by analogy with such expressions as good day, perhaps after people no longer had a clear idea of the original sense of the expression. A letter of 1573 written by Gabriel Harvey contains the first recorded use of goodbye: “To requite your gallonde [gallon] of godbwyes, I regive you a pottle of howdyes,” recalling another contraction that is still used.
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