Macrobius, Saturnalia 1.17.64
“They call Apollo Delphios because he shows obscure things in the clarity of light, from making clear (to deloun) things unseen (aphane). Alternatively, as Numenius thinks, they name him so because he is one and alone. For, he says that in the early Greek language, delphon meant one; for this reason, the word for brother is adelphos, as if to signify ‘not one’ [a-delphos].
ἀπόλλωνα Δέλφιον vocant, quod quae obscura sunt claritudine lucis ostendit, ἐκ τοῦ δηλοῦν ἀφάνῆ, aut, ut Numenio placet, quasi unum et solum. Ait enim prisca Graecorum lingua δέλφον unum vocari: unde et frater, inquit, ἀδελφὸς dicitur, quasi iam non unus.