delta [M16th] The triangular area of sediment at the mouth of some rivers takes the name delta from its shape, which is like that of the triangular fourth letter of the Greek alphabet, called delta. The original delta was at the mouth of the River Nile, which was called the Delta from the mid 16th century. The shape of the Greek letter also gave its name to the delta wing, a triangular swept-back wing fitted on some jet aircraft, immediately after the Second World War.