An only child was an individual with no siblings.
Pavel Chekov was an only child. However, while under the influence of the Beta XII-A entity in 2268, he once erroneously believed that he had had a brother named Piotr who was killed by the Klingons. (TOS: "Day of the Dove")
In 2364, Data told his villainous brother Lore that he (Lore) made him (Data) wish he were an only child. (TNG: "Datalore")
In 2372, Quark responded to Rom's "outrageous" demands on behalf of the Guild of Restaurant and Casino Employees – namely, that Quark rescind the pay cuts he had instituted at Quark's, or regret it – retorting, "The only thing I regret is not being an only child." (DS9: "Bar Association")