A baseball that is thrown by a pitcher and breaks or curves from the expected or straight flight towards home plate is a curveball. It will typically curve to the left when thrown by a right-handed pitcher and to the right when thrown by a left-handed pitcher.
Also known simply as a "curve".
The curve is thrown with a designated grip and hand motion that gives the ball forward spin, causing the ball to dive downward as it reaches home plate. Closely related to the "slurve" and "slider", the curveball can vary greatly from one pitcher to the next. The degree of break on the thrown ball will depend on the force the pitcher uses when snapping the throw. Typically, left-handed pitchers throw curveballs a much higher percentage of the time than right-handed pitchers. A curve is used to deceive the hitter into thinking the ball will come across the plate, when in fact it ends up nowhere near the plate most of the time. Pitchers will also start a curveball off the plate and allow it to curve back over the plate. A good curveball can break nearly two feet.