An Etymological Dictionary of Astronomy and Astrophysics

1. The fourth letter of the Greek alphabet (δ, Δ).
   

2. Anything triangular, like the Greek capital delta (δ).
   

3. Math.: An incremental change in a variable.

See also: M.E. deltha, from L. delta, from Gk. delta; from the Phoenician name of the corresponding letter daleth “tent door.”