The change in an object’s velocity over time is called acceleration. Acceleration is defined as the rate of change of velocity, which can occur due to changes in speed, direction, or both. It is a vector quantity, meaning it has both magnitude and direction. When an object speeds up, it experiences positive acceleration, while negative acceleration, or deceleration, occurs when it slows down. Understanding acceleration is crucial in physics and engineering, as it helps analyze motion, predict an object's future position, and design systems that rely on kinematics.