What is the term for the change in velocity over time?

The term for the change in velocity over time is acceleration.

Acceleration measures how quickly an object's velocity changes, including changes in both speed and direction. Average acceleration is the change in velocity divided by the elapsed time, while instantaneous acceleration is the derivative of velocity with respect to time. Because velocity is a vector, acceleration is also a vector.

The SI unit is the metre per second squared (m/s²). An acceleration of 3 m/s² means that velocity changes by 3 metres per second every second, assuming the acceleration remains constant. Newton's second law connects acceleration with net force through F = ma.

A frequent mistake is treating acceleration as synonymous with speeding up. An object moving at constant speed around a curve is still accelerating because its direction changes. Conversely, slowing down is also acceleration: it is often called deceleration, but physically it is acceleration opposite to the direction of motion. Momentum depends on mass and velocity, while speed is only the magnitude of velocity.