birthday UK /ˈbɜːθ.deɪ/US /ˈbɝθˌdeɪ/noun 4verb 1forms 3phrases 2 noun4 •the date each year that is the same as the date someone was bornWhen's your birthday? Mine's on April 1.I'll be thirty-seven (years old) next birthday. •The anniversary of the day on which something is created. •A birthday party.I'd like to invite you all to my birthday. •The date on which someone is born or something is created, more commonly called birthdate or date of birth. verb1 •(informal) To celebrate one's birthday. formsbirthdayed · birthdaying · birthdays Phrases with birthday birthday suitNakedness, unclothed skin; an exposure of a human's natural anatomy without clothing. birthday-suitedUnclothed.