What is an unforced error in tennis?
Whether an error is forced or unforced is a statistician's judgement, not a rule, so counts vary between sources. A forced error is one the opponent's shot caused. A winner is a shot the opponent doesn't reach.
Questions
What's the difference between a forced and an unforced error?
A forced error comes from pressure the opponent created. An unforced error is a miss on a playable ball.