(idiomatic, often humorous) Nakedness, unclothed skin; an exposure of a human's natural anatomy without clothing. / Used other than figuratively or idiomatically: see birthday, suit.

(cryptography) A covert method of decryption exploiting the so-called birthday paradox.

(probability theory) The mathematics problem that concerns the probability there exist two people in a random group who share a birthday.

plural of birthday boy

plural of birthday card

Said or written to a person who is celebrating his or her birthday in order to convey the good wishes of the speaker or writer (literally, "may you have a happy birthday")

(colloquial, transitive) To wish (someone) a happy birthday.

(US) A public holiday in the USA celebrated on the third Monday of February, during which the US presidents, especially George Washington, are celebrated or remembered.

plural of birthday girl

plural of birthday cake