An error caused by the quality of the opponent's shot rather than the player's own mistake. If someone rips a forehand to the corner and you can barely get a racquet on it, your mishit is a forced error, not an unforced error. The distinction matters in stats because forced errors credit the opponent's aggression, while unforced errors are self-inflicted.
"Incredible forehand from Sinner pulls her wide and the backhand goes into the net. That's a forced error. Nothing she could do with that."