Wiktionary
backward pawn
n. (context chess English) A pawn that is unable to advance without being threatened by an enemy pawn and is not defensible by pawns of the same color.
Wikipedia
Backward pawn
In chess, a backward pawn is a pawn that is behind all pawns of the same color on the adjacent files and cannot be safely advanced. In the diagram, the black pawn on the c6-square is backward.