The curveball is a type of pitch in baseball thrown with a characteristic grip and hand movement that imparts forward spin to the ball causing it to dive in a downward path as it approaches the plate. Its close relatives are the slider and the slurve. The "curve" of the ball varies from pitcher to pitcher. Outside the context of baseball, variants of the expression "to throw a curveball" essentially translate to introducing a significant deviation to a preceding concept.
Read more about Curveball: Grip and Action, The 12–6 Curveball Vs. The "slurve", Physics of A Curveball, Real or Illusion?, Nicknames, History