• acceleration = net force/mass = (applied force - resistive force)/mass

• The resistive force increases with velocity (more often than not, F_resistive µ v²)

• As the speed increases, F_applied  and F_resistive become more and more nearly equal, so the acceleration diminishes (although the speed goes on increasing)

• Eventually F_applied  and F_resistive  become equal, and the acceleration becomes zero. So the velocity is constant, and is often known as the terminal velocity.

This situation can apply to falling stones, falling parachutes, gliders, planes, rowing boats, cars, etc. The source of F_applied might be gravity or an engine, etc, and the source of F_resistive might be air resistance, internal friction, etc.