Isolated pawns are pawns that cannot be supported by any pawns next to them. This does not necessarily mean that they are passed pawns, because they can still have enemy pawns opposing their advance. However, isolated passed pawns are quite common, and can be very strong in the endgame (e.g. outside passed pawn).
Isolated pawns are considered a pawn weakness and can often be attacked. The attacker tries to blockade the pawn and that put as many pieces as possible against it. The defender cannot defend the pawn with other pawns (because its isolated), and must defend it with pieces.
The most common case of isolated pawns is the Isolated Queen Pawn (meaning the "d pawn), which is so common in chess games that it is often abbreviated as IQP. Many chess openings, particularly Queen pawn openings, result in an IQP position.
Doubled pawns can be isolated, and less commonly tripled pawns. Doubled isolated pawns are quite weak, although the one at the rear is usually less weak from the one in front, which provides some cover.
Isolated pawns can also be backward pawns. An isolated backward pawn is a sad fellow that cannot advance and is likely to be attacked.
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