Isolated pawn
A pawn with no friendly pawns on the adjacent files — vulnerable to attack but often controlling key central squares.
An isolated pawn — sometimes called an isolani — is a pawn that has no pawns on the files immediately beside it. Its left and right neighbours are both gone, and there is no possibility of a pawn defending it. Because pawns defend each other so naturally, the isolated pawn is a permanent structural feature; it can never be defended by another pawn for the rest of the game.
The isolated pawn’s weakness is concrete: it must be defended by pieces. Whichever pieces are tied to defending it are not free to attack elsewhere. The square in front of the isolated pawn is also a permanent outpost for the opponent — no pawn can ever drive a piece off that square, because no pawn is on the adjacent files.
The isolated pawn also has a strength. It often controls two central squares and supports pieces in active positions. The classical isolated queen’s pawn (IQP) on d4 — a recurring structure in the Queen’s Gambit and Nimzo-Indian — gives White a piece-active position with attacking chances against the enemy king. The structure has been treated as both an asset and a liability in different eras of opening theory.
Modern engine analysis tends to evaluate isolated pawns slightly worse than older books did. The piece activity that compensates for the weakness is real but rarely sufficient against precise defence. Still, at any level below the very top, an IQP position remains a practical fight with chances for both sides.