'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.'