In chess, an isolated pawn is a pawn for which there is no friendly pawn on an adjacent file. In the endgame, isolated pawns are a weakness because they cannot be defended by other pawns. In this diagram, the pawn on the e5 square is isolated. (Also see backward pawn and doubled pawns.)
There are two reasons isolated pawns are weak.
Firstly is the pieces attacking them usually have more elasticity than those defending them. This means that the attacking pieces are more able to also do other things (threaten to win pieces, checkmate etc) than those that are tied to defending a pawn. A piece that is attacking a pawn can give up the attack to do something else whereas the piece defending must stay rooted to the spot until the attacking piece has moved.
The second reason is that the square immediately in front of the isolated pawn is weak because it is immune to attack by a pawn. Thus an isolated pawn provides a typical example of what steinitz called weak squares.
An isolated queens pawn (IQP) is thought by many to be a special case. An isolated queens pawn is one in the queens file (d file). The weakness of such a pawn consists in its having to be defended and the weak square infront of it (ie d5 for white) being of particular importance. However the presence of open files in the important king and queens bishop (e and c files) as well as the outposts at e6 and c6 enable the player with the IQP very favourable attacking chances in the middle game. Once the game reaches the end game the isolated nature of the pawn becomes a greater weakness than these strengths. Therefore the person with the IQP must take advantage of the temporary strength before an endgame is reached.
Aron Nimzovichīs My System