Definition of birthday
- her 21st birthday
Her birthday is September 30th.
Today is his 21st birthday.
The company just celebrated its 50th birthday.
These example sentences are selected automatically from various online news sources to reflect current usage of the word 'birthday.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.
: the day when someone was born or the anniversary of that day
: the day when something began
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