(intransitive, informal) To celebrate one's birthday. The anniversary of the day on which someone is born. [From 1570s]. The anniversary of the day on which something is created.. The date on which someone is born or something is created, more commonly called birthdate or date of birth.
(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.