True vertical depth
True vertical depth, abbreviated as TVD, is the measurement of a straight line perpendicularly downwards from a horizontal plane.
In the petroleum industry true vertical depth is the measurement from the surface to the bottom of the borehole in a straight perpendicular line represented by line in the image.
Line is the actual borehole and its length would be considered the "measured depth" in oil industry terminology. The TVD is always equal to or less than the measured depth. If one were to imagine line to be a piece of string, and further were to imagine it being pulled straight down, one would observe it to be longer than line. This example oil well would be considered a directional well because it deviates from a straight vertical line.