When a small metallic sphere is gently placed on the surface of a highly viscous liquid it descends down with an acceleration. As its velocity increases, the viscous force opposing the motion of the sphere also increases.
At some stage the apparent weight of the sphere becomes equal to the force due to viscous drag.
So the resultant force on the sphere is zero. Therefore the sphere moves down with a constant velocity known as terminal velocity.
Terminal velocity is that velocity at which the apparent weight of the sphere is equal to the viscous force acting on the sphere moving in a high viscous liquid.