[noun] [C] - the day that is an exact year or number of years after a person was bornHappy birthday (to you)!Are you going to Ellen's birthday party next week?It's her 21st birthday.What did you get for your birthday (= What present(s) did you receive)?(informal humorous) If you are wearing/in your birthday suit you have no clothes on.
BIRTHDAY
Meaning of BIRTHDAY in English
Cambridge English vocab. Кембриджский английский словарь. 2012