The rate of change of velocity with respect to time is called acceleration. Mathematically, it is defined as the derivative of velocity with respect to time. In symbols, acceleration (a) can be expressed as:
a = d(v) / dt
Where: a is acceleration, v is velocity, and dt is the infinitesimally small change in time.
In simpler terms, acceleration measures how quickly the velocity of an object is changing over time. If the acceleration is positive, it means the velocity is increasing, while a negative acceleration indicates the velocity is decreasing. Acceleration can also be zero if the velocity remains constant over time.