Let's consider a body (that we will call the *impactor*)
that travels in a certain direction. This body is headed to
impact with a second body (called *target body*). The
bigger the target is, the more it will have a chance of being hit
by the impactor.

This simple concept can be formalized introducing the *geometrical
cross section*, which is the area occupied by the target
body projected on a plane orthogonal to the direction of the
motion. Therefore, this *cross section* is proportional to
the chances of a hit. If the target body
has a non geometrical shape, its cross section will be the area
projected on the plane orthogonal to the direction of the motion.