A large downward-pointing triangle operator commonly used to denote gradient descent or minimization in optimization theory.
Examples
Del operator in vector calculus (nabla/gradient)
\bigtriangledown f(x,y,z) = \frac{\partial f}{\partial x}\mathbf{i} + \frac{\partial f}{\partial y}\mathbf{j} + \frac{\partial f}{\partial z}\mathbf{k}
Divergence operator in vector calculus
\bigtriangledown \cdot \mathbf{F} = \frac{\partial P}{\partial x} + \frac{\partial Q}{\partial y} + \frac{\partial R}{\partial z}