A piece attacks two pieces at once.
A piece attacks a piece on the same diagonal or line as the opponent's King or a second, higher-value piece.
A piece attacks the opponent's King or higher-value piece that is on the same diagonal or line as a second piece. (opposite of the Pin)
A version of double attack where you attack an opponent's piece AND threaten mate-in-one.
Moving a piece that was blocking an attack from your second piece, thus 'uncovering' the attack on the opponent's piece or King.