In the oil and gas industry, depth in a well is the distance along a well between a point of interest and a reference point or surface. It is the most common method of reference for locations in the well, and therefore, in oil industry speech, "depth" also refers to the location itself.
Strictly, depth is a vertical coordinate related to elevation, albeit in the opposite direction. However, "depth" in a well is not necessarily measured vertically or along a straight line.
Because wells are not always drilled vertically, there may be two "depths" for every given point in a wellbore: the measured depth (MD) measured along the path of the borehole, and the true vertical depth (TVD), the vertical distance between the datum and the point of interest. In perfectly vertical wells, the TVD equals the MD; otherwise, the TVD is less than the MD measured from the same datum.
Common datums used are ground level (GL), drilling rig floor (DF), Rotary table (RT), kelly bushing (KB or RKB) and mean sea level (MSL).