1. Relating to the day of one's birth.

2. The day of one's birth, or its recurrence.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• birthhday,
• birithday,
• birthay,
• birthdy,
• berthday,
• birrthday,
• birthdaty,
• biirthday,
• birthdat,
• earthday,
• birsthday,
• birtyday,
• bithday,
• birtghday,
• brithday,
• birtday,
• birthjday,
• abirthday,
• birthady,
• birthsday

The adjective form of the word "birthday" is "birthday".

The plural of the "birthday" can be the "birthdays".

The singular of the "birthday" can be the "birthday".