I. noun 1. the anniversary of the day on which a person was born, typically treated as an occasion for celebration and the giving of gifts. • his twenty-ninth birthday.
• [as modifier]
a birthday cake.
2. the day of a person’s birth • he shares a birthday with Rod Stewart.
3. the anniversary of something’s origin or foundation • the seventy-fifth birthday of the Institute.
II. phrases in one’s birthday suit