1. Also called line' of sight'ing. 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.