Terminal velocity is the constant speed achieved by an object freefalling through a gas (eg air) or liquid. Terminal velocity is therefore reached when its speed is no longer increasing or decreasing – ie the drag force and buoyancy are equal to the downward force of gravity – with the net force acting on it balancing out at zero.
The two main factors that dictate an object’s terminal velocity on Earth are its weight and surface area, with heavier, small surface area objects having a greater velocity. For example, a lead ball will have a much higher terminal velocity than a sheet of paper as the former both weighs more and occupies less space.
The importance of surface area is due to the gas or liquid medium’s drag effect. For example, the air in Earth’s atmosphere generates resistance due to its molecules colliding into any falling body and creating an upward force in opposition to gravity. This is why if two differently weighted objects are dropped into a vacuum at the same speed, they will experience the same acceleration (as shown in the famous feather/hammer drop test conducted on the Moon during the Apollo 15 mission).