The terminal velocity of an object is the highest velocity it can reach when falling through a defined medium, usually air in practice. This constant velocity is achieved when the net force applied to the object is zero, this means that the resistance is equal to the force being applied by gravity. To put this into a real life example, if you were to throw a ball off a helicopter it would accelerate at a decreasing rate until it reached a speed so high that the air resistance it was experiencing was equal to the force that gravity applied to it. Applying Newton's 1st and 2nd laws, we can work out that this means it will continue at an unchanged velocity until its hits the ground.There are multiple factors which affect an objects terminal velocity. These include its weight and aerodynamics. The terminal velocity will also be different in different fluids. For example any object will have a lower terminal velocity in water than in air due to its higher density.