plural birthdays
1
: the day when someone was born or the anniversary of that day
Her birthday is September 30th.
Today is his 21st birthday.
Happy Birthday!
—often used before another nouna birthday party/gift
2
: the day when something began
The company just celebrated its 50th birthday.
Cite this Entry
“Birthday.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/birthday. Accessed 15 Apr. 2026.
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