A woman or girl whose birthday it is.

A man or boy whose birthday it is.

A greeting card given to someone to celebrate his or her birthday.

A cake, often iced and decorated, made to celebrate a person's birthday.

(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")