delta

Concise Oxford English Dictionary © 2008 Oxford University Press:
delta1

▶noun

• 1 the fourth letter of the Greek alphabet (Δ, δ), transliterated as ‘d’.

  ■ [as modifier] denoting the fourth in a series of items or categories.

  ■ Brit. a fourth-class mark given for a piece of work.

• 2 a code word representing the letter D, used in radio communication.

▶symbol Mathematics variation of a variable or quantity.

– origin Gk, from Phoenician daleth.

Concise Oxford English Dictionary © 2008 Oxford University Press:
delta2

▶noun a triangular tract of sediment deposited at the mouth of a river, typically where it diverges into several outlets.

– derivatives
deltaic adjective.

– origin C16: orig. the Delta (of the River Nile), from the shape of the Greek letter (see delta1).