Upside-down triangle symbol on function

Question

I came across the symbol that looks like an upside-down triangle, and coming in front of a function $f(x,y)$. What does that mean?

Answer 1

The upside-down triangle is likely a gradient operator in this context:

$$\nabla f(x,y) = \frac{\partial f}{\partial x}\hat{x} + \frac{\partial f}{\partial y}\hat{y}.$$