Isolated pawn
An isolated pawn has no friendly pawn on either neighboring file, so it cannot be protected by another pawn.
The isolated queen’s pawn (IQP) on d4 is the textbook case. It grants space and open files for the pieces, and it is also a fixed target that Black can blockade on d5.
White has no c-pawn and no e-pawn. The d4 pawn must be guarded by pieces. Black’s c5 pawn attacks it, which is the usual way to pressure an IQP.
Whether the pawn is a strength or a weakness depends on the pieces. If White’s knights and rooks are active, the extra space matters. If they get traded, the ending is often just a weak pawn.