Isolated pawn

In chess, an isolated pawn is a pawn for which there is no friendly pawn on an adjacent file. An isolated queen's pawn is often called an isolani.In the endgame, isolated pawns are a weakness because they cannot be defended by other pawns. In this diagram, the white pawn on the e5 square and the black pawn on a7 are isolated. (Also see backward pawn and doubled pawns.)Isolated pawns are weak for two reasons. First of all, the pieces attacking them usually have more elasticity than those defending them. In other words, the attacking pieces enjoy greater freedom to do other things (threaten to win pieces, checkmate, etc.), while the defending pieces are restricted to the defense of the pawn. This is because a piece that is attacking a pawn can give up the attack to do something else whereas the defending piece must stay rooted to the spot until the attacking piece has moved. The defending piece is thus said to be "tied down" to the pawn.
See more at Wikipedia.org...