Line of sight (missile)
In missile guidance, Line of sight is the straight line between the missile and the target. At the end of the engagement the distance will be zero. See Command guidance#Command to Line-Of-Sight.
By Pursuit guidance the missile is steered so that the velocity vector of the missile always points at the target, i.e. has the direction of the Line of sight. Proportional navigation tries to maintain a constant LOS.