1. Also called line of sighting . an imaginary straight line running through the aligned sights of a firearm, surveying equipment, etc.
2. Astron. an imaginary line from an observer to a celestial body, coincident with the path traveled by light rays receivedfrom the body.
3. Radio. a straight line connecting two points sufficiently high and near one another so that the line is entirely above the surface of the earth.
4. Ophthalm. See line of vision .
5. sightline.
[ 1550-60 ]