In the oil industry measured depth (commonly shortened to MD), is the length of the borehole

Borehole

A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...

, line (b) in the image, and is measured in feet or metres. The borehole in this image is deviated from the vertical but many oilwells are still just conventional vertical wells (straight holes).

Measured depth is normally referred to as just depth except when the oilwell is directional

Directional well

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 :...

 (as is the case in the image) or horizontal. In these wells, certain engineering calculations depend upon the true vertical depth

True vertical depth

True vertical depth is the measurement of a straight line perpendicularly downwards from a horizontal plane.In the petroleum industry, true vertical depth, abbreviated as TVD, is the measurement from the surface to the bottom of the borehole in a straight perpendicular line represented by line in...

, line (a), to determine certain fluid mechanics

Fluid mechanics

Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...

.

The true vertical depth

True vertical depth

True vertical depth is the measurement of a straight line perpendicularly downwards from a horizontal plane.In the petroleum industry, true vertical depth, abbreviated as TVD, is the measurement from the surface to the bottom of the borehole in a straight perpendicular line represented by line in...

is always equal to or less than (≤) the measured depth. If you imagine line (b) were a piece of string and pull it straight down, you would see that it would be longer than line (a).