Boresight is also a term used to describe crude adjustments made to an optical firearm sight, or iron sights, to align the firearm barrel and sights. This method is usually used to pre-align the sights, which makes zeroing (zero drop at XX distance) much faster.
Traditional boresighting, as the name suggests involves removing the bolt and sighting down the bore of a gun to a fixed point. While the rifle is fixed in place, the scope or irons can then be adjusted to also aim at the distant object. A more modern method of boresighting is to use a laser to illuminate the distant point, rather than by visual inspection. This method is preferable because it allows more movement in the gun, as the laser dot will not move relative to the barrel, and is a method of boresighting, which does not require the removal of the bolt. A more advanced method of boresighting, uses a collimeter, which is an optic attachment, similar to a scope sight, which fit onto the end of the barrel. Using this method, the normal sight, which is fixed to the stock, and the collimeter, which is fixed to the barrel can be sighted to match. Most collimeters have grid patterns for rechecking the zero after the barrel is sighted.
Boresight is also a term used to describe the accurate alignment of avionic equipment in an airframe or other flight vehicle (e.g. head-up displays, inertial reference systems, guidance systems, radar and other sensor systems, weapon systems, etc.).
Accuracy is the measure of how well the sighted object is represented. It can be measured from a specific decisionmaking circumstance, like the orientation of notches of a gun barrel. Alternately, the device could be designed to accommodate a range of circumstances and still be sufficiently accurate. Heads-up displays are independent of eye position over a pre-set range, for example.
It may be by electronic offset adjustment or by physical—mechanical—alignment of the equipment mountings, allowing equipment replacement without re-boresighting, hence the term "line replacable unit". This concept is, among other things, instrumental in reducing many flight delays.