birth‧day S2 W3 [countable]
your birthday is a day that is an exact number of years after the day you were born:
He was in Kansas to celebrate his 74th birthday.
happy birthday! (=what you say on someone's birthday)
birthday card/gift/party etc
Anne's birthday party
the birthday girl/boy informal (=the person whose birthday it is)
a day that is an exact number of years since an organization was established or an event first happened [↪ anniversary]:
The City of Cleveland Orchestra is celebrating its 200th birthday.
not wearing any clothes - used humorously