| Dictionary entry | Language | Definition |
|---|---|---|
| birthday | English (eng) | (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. |
| birthday paradox | English (eng) | (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. |