(statistics) A statistical phenomenon where an individual's likelihood of death appears to increase on or close to their birthday, variously attributed to alcohol consumption, psychological stress, increased suicide risk, and other factors.

plural of birthday party

Unclothed.

plural of birthday attack

(rare) An unclothed person.

(historical) A book, in diary form, in which one's friends enter their birthdates in their own handwriting.

plural of birthday book

A birthday of a person after his or her death.

(probability theory) The counterintuitive observation that a random group of people needs to have only 23 members before there is a 50% chance of two of them having the same birthday.

plural of birthday suiter