where v is the velocity, and t is the time. This slope therefore defines the average acceleration over the interval, and reducing the interval infinitesimally gives d v d t , the instantaneous acceleration at time t , or the derivative of the velocity with respect to time (or the second derivative of the position with respect to time). In SI, this slope or derivative is expressed in the units of meters per second per second ( m / s 2 , usually termed "meters per second-squared").