Δ

Symbol

 * : change in a variable
 * 1)  Used on the reaction arrow in a chemical equation, to show that energy in the form of heat is added to the reaction
 * 2)  Used to represent the symmetric difference (also known as the disjunctive union) of two sets
 * 3)  Used before a gene name as a modifier to denote a deletion mutation of that gene.
 * 4)  Used after a pitch class to denote a major seventh chord with a note of said pitch as its root.

Usage notes
Unicode establishes a distinct symbol (∆) for particular use in mathematics, but Δ is often substituted for convenience.

Letter

 * 1) The 6th letter of the Arvanitic Albanian Greek-script alphabet.

Etymology
From the letter.

Letter

 * 1) The letter delta, the fourth letter of the ancient Greek alphabet.

Letter

 * 1) The upper case delta (δέλτα), the fourth letter of the modern Greek alphabet, representing a voiced interdental fricative.