In land surveying, an instrument used to measure angles and approximate distances. Transits typically consist of a telescope, with accurate cross hairs, mounted on a ring with vernier scales that measure angles in degrees and minutes (a minute is a sixtieth of a degree).
The scales can be locked and unlocked, allowing angles to be "turned" multiple times in multiple directions to check for error and squeeze the maximum accuracy out of the instrument.
To measure distances, two particular marks on the cross hairs of the transit are used. Above and below the center, along the vertical cross hair are two additional horizontal marks. These have been set such that they mark off a very particular angle. At a distance, if the pole being looked at has a single foot falling between the two hairs, then the pole is 100 feet away (usually, a level rod is used for this purpose, since it has feet and tenths marked off on it). If it has 2.34 feet falling between the hairs, it is 234 feet away, and so on. The process of measuring distances this way is only accurate to about 1 part in 100, but this is good enough for certain types of activity. Called "shooting stadia", this is a very quick way to measure distances when great accuracy is not required (such as a "topo" survey of a yard, where features such as trees and bushes are marked on a map).
In modern circumstances, transits are only used for short distances and work that doesn't require great accuracy. They've been supplanted by theodolites that measure angles much more accurately.
See: land surveying.