A sole male child among multiple siblings. / Used other than figuratively or idiomatically: see only, son.

plural of only son

plural of only daughter

(espionage) Too secret to be put in writing; only to be transmitted orally.

(espionage, of documents) Meant to be read only, and not discussed, by the named recipient; classified.

plural of one and only

(idiomatic) Unique, one of a kind.

(idiomatic) Only love, sole sweetheart.

(travel, UK, Ireland, dated) A package holiday rate without confirmed accommodation reservations, or using a dummy reservation, enabling the tour operator to sell charter airline seats in competition with the airlines.

Being the only child of one’s parent (especially one’s father) / (Christian theology) One of the attributes of Jesus Christ, the only-begotten Son of God the Father.