That upside down triangle is the nabla symbol and is typically called "del". "Del" is an operator analagous to the derivative operator d/dx except that del takes partial derivatives. In Cartesian 3-space,
\boldsymbol{\nabla} \equiv<br />
\hat{\boldsymbol x} \frac{\partial}{\partial x} +<br />
\hat{\boldsymbol y} \frac{\partial}{\partial y} +<br />
\hat{\boldsymbol z} \frac{\partial}{\partial z}<br />
When applied to a scalar function f(x,y,z), the del operator yields the gradient of the function:
\boldsymbol{\nabla} f(x,y,z) \equiv<br />
\hat{\boldsymbol x} \frac{\partial f(x,y,z)}{\partial x} +<br />
\hat{\boldsymbol y} \frac{\partial f(x,y,z)}{\partial y} +<br />
\hat{\boldsymbol z} \frac{\partial f(x,y,z)}{\partial z}<br />
The operator definition of del looks like a vector. With a little abuse of notation, it can be applied to vector functions as a dot product (yielding a scalar) and a cross product (yielding a vector):
\boldsymbol{\nabla} \cdot \boldsymbol{f}(x,y,z) \equiv<br />
\hat{\boldsymbol x} \frac{\partial f_x(x,y,z)}{\partial x} +<br />
\hat{\boldsymbol y} \frac{\partial f_y(x,y,z)}{\partial y} +<br />
\hat{\boldsymbol z} \frac{\partial f_z(x,y,z)}{\partial z}<br />
and similarly for the cross product. The expression \boldsymbol{\nabla} \cdot \boldsymbol{f}(x,y,z) is called the divergence of f(x,y,z) while \boldsymbol{\nabla} \times \boldsymbol{f}(x,y,z) is called the curl.
