What is terminal velocity?

Terminal velocity is the final velocity an object achieves during freefall through a medium under the influence of gravity and upthrust from the medium. For example, dropping a tennis ball from a plane. The velocity at the instantaneous start will only be caused by the weight force of the ball. As it falls, it gains velocity due to gravitational acceleration, but the frictional force due to air resistance also increases. As velocity of the ball increases, the force due to friction also increases. Once the force from friction is equal to the weight force of the ball, the velocity of the ball will reach a maximum and stay constant. The assumption here is that there are no other forces acting on the ball other than the gravitational force and the frictional force from air resistance.