Sightline
Also said as Line of sight, LOS, Sight lane
How far and where a player can see, and be seen, from a given spot. In a shooter, a sightline is also a line of fire: if I can see you, I can hit you.
A long clear line favours one weapon and one kind of player. A level with none feels cramped. Which sightlines exist is the most important decision in a multiplayer map, and players find any you did not mean to leave in.
Say it
“That corridor gives the sniper rifle 80 m of clear line right into spawn. Add a pillar a third of the way along, so nothing has more than 30 m.”
Where you’ve seen it
On Dust II in Counter-Strike, a sniper can see straight down mid through the double doors, and crossing it is a known risk. Teams throw a smoke grenade there to cut the line.
Not to be confused with draw distance
Draw distance is how far the engine bothers to render. A sightline is about what the walls allow. A wall ends a sightline no matter how far the engine could draw.