∇×F
Where F is the vector field the curl is being applied to, and is composed of [Fx, Fy, Fz].
Expanded, ∇×F looks like:
[∂Fz/∂y - ∂Fy/∂z, ∂Fz/∂x - ∂Fx/∂z, ∂Fy/∂x - ∂Fx/∂y]
A simple way to remember the expanded form of the curl is to think of it as:
[∂/∂x, ∂/∂y, ∂/∂z]×F
or as the determinant of the following matrix:
i | j | k |
∂/∂x | ∂/∂y | ∂/∂z |
Fx | Fy | Fz |
where i, j, and k are the unit vectors for the x, y, and z axes, respectively.
Note that the result of the curl operator is not really a vector, it is a pseudovector?. This means that it takes on opposite values in left-handed and right-handed coordiante systems.