A square that cannot be easily defended from attack by an opponent. Often a weak square is unable to be defended by pawns (a hole). Exchange or loss of a bishop may make all squares of that bishop's color weak resulting in a "weak square complex" on the light squares or the dark squares.
WSQ: Weak Square for a side is a strong square for the opponent (Current Chess Theory - Chess Terminology).
a square that cannot easily be defended, and may become a hole or outpost for the opponent. There are examples in the Strategy/themes section of the Canon.