1. n. [Drilling]

The angle measured in a plane perpendicular to the drillstring axis that is between a reference direction on the drillstring and a fixed reference. For near-vertical wells, north is the fixed reference and the angle is the magnetic toolface. For more-deviated wells, the top of the borehole is the fixed reference and the angle is the gravity toolface, or high side toolface.

See: borehole orientation,  gravity toolface,  high-side toolface,  magnetic toolface,  wellbore orientation