delta (n.)

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.