File:True vertical depth.jpg
Depiction of a directional well.
Line (a) is an imaginary line representing the True vertical depth, while line (b) is the borehole itself, and its length is called the measured depth

A directional well, as it is known in the oil industry, is an oil well with a borehole that deviates from a vertically straight line. This is normally an intentional decision and done purposefully, to hit several target sands, for instance.

References