1. or line of sighting : a linear projection toward a target of the straight line obtained when the sights of a firearm are in perfect alignment : the axis of the line of vision of a sight (as of an artillery piece) or other angle-measuring instrument
2. : line of vision
3.
a. : a line from an observer's eye to a distant point (as on the celestial sphere) toward which he is looking or directing an observing instrument
b. : a line joining the earth or the sun and a distant astronomical body
4. : the straight path between a radio transmitting antenna and receiving antenna when unobstructed by the horizon
reports of receptions far beyond the line of sight beyond which the signals should not have gone — Science News Letter