'Del' is the directional derivative operator from vector calculus. It is represented by an upside-down 'delta' like so: ∇.

In cartesian coordinates in three dimensions,
= (i ∂/∂x + j ∂/∂y + k ∂/∂k)
where i, j, and k are the unit vectors in the x, y, and z directions respectively, and ∂/∂w is the partial derivative with respect to w.

Del operates on what is to the right of it. For instance,
s = i ∂s/∂x + j ∂s/∂y + k ∂s/∂k, and is called the gradient of s.

As del has components, it acts like a vector, and so the product can be taken with del and a vector. For instance the dot product of del with a vector is called the divergence
v = ∂s/∂x + ∂s/∂y + ∂s/∂k and this is a scalar quantity. The cross product of del with a vector is called the curl.

More information on del and its properties and uses is available under vector calculus. Del can also be called 'nabla.'