pawns not defended and not defensible by the pawns of the same color, whose stop square is also not covered by a friendly pawn. Some programs, most notably Crafty, do not differentiate between various classes of weak pawns, but most of the time their various kinds are evaluated differently.
Classes of Weak Pawns
- Isolated Pawn
- Backward Pawn
- Overly advanced pawn (technically neither isolated nor backward, but nevertheless not defensible by a pawn. Consider the following situation: white pawns on a3, b5 and c4, black pawns on a4, b6 and c7, where the black pawn on a4 falls into this category.)
1k6/2p5/1p6/1P6/p1P5/P7/3K4/8 w - -
Some also add hanging pawns, but they are not as endangered and have a lot more attacking potential than any of the above.