The rate of increase of velocity with time. More specifically, the derivative of the velocity vector with respect to time, dv/dt. Objects accelerate when they are subject to a force. Note that accelerations can be both positive and negative (negative accelerations are often referred to as decelerations), and can have a different sign (or direction) from an object's velocity. Acceleration is measured in length per second squared. A commonly used unit of acceleration is g, the acceleration due to gravity at the surface of the Earth (equal to 9.8 meters per second squared).
Accelerated motion stands in contrast to Inertial Motion, or constant velocity motion.