A capital Greek delta symbol representing change, difference, or the Laplacian operator in calculus and physics.
Examples
Change in a variable
\Delta x = x_2 - x_1
Discriminant in quadratic formula
\Delta = b^2 - 4ac
Laplace operator in vector calculus
\Delta f = \frac{\partial^2 f}{\partial x^2} + \frac{\partial^2 f}{\partial y^2} + \frac{\partial^2 f}{\partial z^2}