n[C]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 Are you going to Anne's birthday party?the birthday girl/boyinformal (=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
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anniversary The City of Cleveland Orchestra is celebrating its 200th birthday .in your birthday suitinformalnot wearing any clothes - used humorously