A backhand is when you hit a ball with a racket or paddle with your arm across your body and the back of your hand facing the ball. Some tennis players find it challenging to hit a backhand.
You can use the word backhand as a noun or an adjective, to describe a stroke in tennis, squash, or any other sport that involves a racket. When backhand is a verb, it can mean to hit the ball this way, but it more often means to hit or slap someone with the back of your hand. The tennis stroke meaning has been around since the 1650s.