The terminal velocity is way of expression or calculation of free falling body/mass/object(with out any internal force)at constant speed where it eventually experiance the resistance of the medium( air/liquid) through which it is falling prevents further acceleration/motion The free falling/projecting towards the gravity through the atmosphere(air) will enconter 2 opposite force act as a external forces which is opposite to each other. The first force is the gravitational force /downward force is due to gravity; the other force is the air resistance's/drag resistance's due to air medium(atmosphere resistance's). The terminal velocity can be described by Newton's second law of motion if the object/mass is in a constant speed F = (m) x (a) F= force / m=mass / a=times acceleration