Backward pawn
A pawn that has fallen behind its neighbours on adjacent files and cannot safely advance — a permanent weakness.
A backward pawn is one that lags behind the pawns on its adjacent files and cannot advance because the square in front of it is controlled by enemy pawns. It cannot be defended by another pawn moving forward, and it cannot move forward itself without being captured. Its weakness is permanent in the strong sense of the word: nothing in normal pawn play can rescue it.
The square in front of a backward pawn is even more important than the pawn itself. That square is an outpost for an enemy piece — typically a knight — and the pawn beneath the outpost cannot drive the piece off. A knight on that outpost is permanent, and the pawn it sits on is a target.
The minority attack in the Queen’s Gambit Declined Exchange Variation is designed specifically to create a backward c-pawn for Black. White advances the b-pawn from b2 to b5, exchanges it for Black’s c-pawn, and leaves Black with a backward pawn on c7 (or sometimes on c6 after structural exchanges). The endgame conversion is then a matter of attacking this single weakness.
Defending a backward pawn often requires giving up the pawn for some other advantage — exchanging it for an enemy pawn that arrives on the wrong square, or pushing it as a sacrifice to free pieces. Holding the pawn passively through the entire game rarely works.