delta

\ \ [16] Delta was the fourth letter of the Greek alphabet, corresponding to English d. Its capital form was written in the shape of a tall triangle, and already in the ancient world the word was being applied metaphorically to the triangular deposit of sand, mud, etc which forms at the mouth of rivers (the Greek historian Herodotus, for instance, used it with reference to the mouth of the Nile). Greek acquired the word itself from some Semitic language; it is related to dāleth, the name of the fourth letter of the Hebrew alphabet.