The path of vision between two locations on a game-map. Used primarily in tactical and battle games, line of sight is used for ranged combat. That is, attacking a unit that is not adjacent to the firing unit.
Every game handles it differently, but the main thing is that you can't fire on something you can't see. So rules are made to take into acount terrain that blocks the view from one hex to another.
A simple example. Imagine a row of three hexes. If all three are open ground with no elevation changes, a unit in the first hex can generally see a unit in the third. But if the middle hex has woods or a building in it, that hex will block line of sight between the outside hexes.
LOS example from Commands and Colors: Napoleonics (GMT)
