An imaginary ray emanating from the tip of an Icehouse piece. If the piece is upright, weird, and/or ungrounded, this line is perpendular to the base of the piece. If the piece is flat and grounded, this line is perpendicular to the base line of the side of the piece that is resting on the playing surface.
In Icehouse, if an attacker's line of sight (also called the "attack line") intersects a defensive piece of another color, the attacker is attacking that piece. If an attacker's line of sight intersects any other kind of piece, it is attacking nothing. The attack line does not penetrate any piece to touch another piece behind the first one.
In Zendo, if a piece's line of sight intersects any other piece, it is said to be pointing at that other piece. Some masters say the line of sight penetrates pieces and some masters say it does not. It is often worth asking about.