Isolated pawn
A pawn with no pawn of the same color on either neighboring file.
An isolated pawn is weak if it is blockaded and the square in front of it can be profitably occupied by an enemy piece. In modern tournament practice, though, players often allow an isolated pawn in their own camp if the squares it controls can be used to advantage.
The strengths and weaknesses of an isolated pawn show clearly in positions from the Queen's Gambit Accepted, where White's isolated d4-pawn is both a weakness and a strength, depending on how active the pieces are.