Delta (/ˈdɛltə/;[1] uppercase Δ, lowercase δ or 𝛿; Greek: δέλτα, délta, [ˈðelta])[2] is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. It was derived from the Phoenician letter dalet 𐤃.[3] Letters that come from delta include Latin D and Cyrillic Д.

A river delta (originally, the delta of the Nile River) is so named because its shape approximates the triangular uppercase letter delta. Contrary to a popular legend, this use of the word delta was not coined by Herodotus.[4]