• A change or difference between mathematical values:
  

• A macroscopic change in the value of a variable in mathematics or science.
  
• Any of the delta particles in particle physics.
  

The lower-case letter δ is used as the symbol for:

• A microscopic change in the value of a variable in mathematics or science.
  
• The Kronecker delta in mathematics.
  
• The Dirac delta function in mathematics.
  
• Deflection in engineering mechanics
  
• A mark for deletion in proofreading. The usage is said to date back to classical times.
  

Letters that arose from Delta include the Roman D and the Cyrillic letter Д.

River Delta

The concept of the delta may perhaps be clarified by further exposition. Webster furnishes the following definition: (1) Delta is the name of the fourth letter of the Greek alphabet (equivalent to the English D) from the Phoenician name for the corresponding letter. The Greeks called the alluvial deposit at the mouth of the Nile, from its shape, the Delta of the Nile. (2) A tract of land shaped like the letter "delta," especially when the land is alluvial, and enclosed within two or more mouths of a river, as the Delta of the Ganges, of the Nile, of the Mississippi" .

Fingerprint Delta, John Edgar Hoover, FBI, The Science of Fingerprints Classification and Uses