A borehole is the generalized term for any narrow shaft drilled in the ground, either vertically or horizontally.

A borehole may be constructed for many different purposes including the extraction of water or fluid (such as oil) or gases (such as natural gas or methane), as part of a geotechnical investigation or environmental site assessment, for mineral exploration, or as a pilot hole for installing piers or underground utilities. Boreholes are also used as water wells.

Typically, a borehole used as a well is completed by installing a vertical pipe (casing) and well screen to keep the borehole from caving. This also helps prevent surface contaminants from entering the borehole and protects any installed pump from drawing in sand and sediment. When completed in this manner the borehole is then more commonly called a well: whether it is a water well, oil well or natural gas extraction well.

Installation
Boreholes may be drilled using a drilling rig, or by a hand-operated rig. The machinery and technique to advance a borehole varies considerably according to manufacturer, geological conditions, fluid to be extracted, and job specification.