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Webster 1913 Edition

Noun.

The night in which a person is born; the anniversary of that night in succeeding years.

The angelic song in Bethlehem field,
On thy

birthnight

, that sung thee Savior born.

Milton.

Noun.

[birth and night.] The night in which a person is born; and the anniversary of that night in succeeding years.

birthnight ‎(plural birthnights)

The night of someone's birth.
The anniversary of that night in subsequent years.
The angelic song in Bethlehem field, / On thy birthnight, that sung thee Saviour born. — Milton.