delta1
  
  n.
  - 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.
 - a code word representing the letter D, used in radio communication.
 
symb.
 Mathematics  variation of a variable or quantity.
 
  Etymology
  Gk, from Phoenician daleth.
 
  
    
delta2
  
  n.
 a triangular tract of sediment deposited at the mouth of a river, typically where it diverges into several outlets.
 
  Derivative
  
  Etymology
  C16: orig. the Delta (of the River Nile), from the shape of the Gk letter (see delta1).