"A square is weak when it is controlled by your opponent and you have little or no chance of regaining control due to a lack of pieces which can effectively fight for that square. Typically, pawns that could have controlled the square have moved past it and, of course, cannot move backwards to help guard it. Single weak squares are called “holes”. Holes on the opponents half of the board that can be occupied by one of your supported pieces are called “outposts”."
"A square is weak when it is controlled by your opponent and you have little or no chance of regaining control due to a lack of pieces which can effectively fight for that square. Typically, pawns that could have controlled the square have moved past it and, of course, cannot move backwards to help guard it. Single weak squares are called “holes”. Holes on the opponents half of the board that can be occupied by one of your supported pieces are called “outposts”."
http://www.chess.com/article/view/what-are-quotweak-squaresquot