1. In baseball, a ball that curves in its trajectory toward home plate. He's a good batter, but he has a hard time hitting a curve ball.

2. An unexpected occurrence or thing that causes confusion or uncertainty. I thought I had confirmed my hypothesis, so these results are a real curve ball.

3. A deceptive action or thing. I don't know, I think her sudden displays of affection are just a curve ball to get me to trust her.