A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are used mainly for surveying applications, and have been adapted for specialized purposes such as meteorology and rocket launch.

A modern theodolite consists of a movable telescope mounted within two perpendicular axes: the horizontal or trunnion axis and the zenith axis. A theodolite measures vertical angles as angles between the zenith, forwards or plunged—typically approximately 90 and 270 degrees. When the telescope is pointed at a target object, the angle of each of these axes can be measured with great precision, typically to milliradian or seconds of arc.