peninsula south of Thrace, from Greek khersonesos "peninsula," from khersos "dry land" + nēsos "island," also "(flooded) land near a river, alluvial land," which is of uncertain origin; traditionally from PIE root sna- "to swim," but this is now generally rejected. "As words for 'island' differ from language to language, [nēsos] is probably an Aegean loan (note that Lat. insula is also of unclear origin)" [Beekes]. Compare isle.
c. 1200, name of the fourth letter of the Greek alphabet (equivalent to our D), which was shaped like a triangle. The name is from Phoenician daleth "tent door." Sense of "triangular island or alluvial tract between the diverging branches of the mouth of a great river" is because Herodotus used it of the delta-shaped mouth of the Nile. It was so used in English from 1550s and applied to other river mouths, of whatever shape, by 1790. Related: Deltaic; deltification.