∆

Etymology
From the capital letter.

Symbol

 * 1)  An increment or change in a variable.
 * ∆x  the change in variable x
 * 1)  The symmetric difference symbol.
 * 2)  The Laplace operator.

Usage notes

 * Although this is the official encoding for the mathematical character, most math publications that apply Unicode (LaTeX publications rarely use Unicode) use simply the Greek letter Δ for convenience.