Definition from Wiktionary, the free dictionary
backsight (plural backsights)
- The rear sight of a firearm.
- (surveying) A measurement or reading taken back towards a point of known elevation, used to calculate the height of the surveying instrument (theodolite, transit, total station).
- (surveying) A measurement of a previously shot point, used to set the angle to zero when occupying a new position.
- (surveying) To shoot a backsight.