Terminal Velocity:

When a body falls through a liquid or gas, it carries along with the layer of the fluid in contact and thus tends to produce a relative motion between the layers of the fluid. This relative motion is opposed by the force due to viscosity. The opposing force increases with the velocity of the body. If the falling body is small the opposing force due to the viscosity of the fluid soon becomes equal to the driving force producing the motion of the body. The body then falls with a constant velocity. This velocity is known as terminal velocity.