What exactly is an unforced error?

Question

I always thought it was when the ball went out, hit the net or anything like that, but I am watching the final right now and these girls have like 7 unforced errors so I am guessing what I thought is an unforced error is not true.

Answer 1

a mistake that you caused by incompetence rather than opponent pressures....

Answer 2

this might help

http://www.tennis.com/yourgame/asktheedi...