When an object reaches its terminal velocity, it can no longer accelerate, so its acceleration becomes zero, and it falls at a constant speed. As an object falls freely through the air, it has two forces acting upon it: gravity and drag.
Every object has a terminal velocity, which is the speed it can travel while having a constant force. When an object falls it typically accelerates as it falls, however, eventually the air resistance or drag becomes too great to allow the object to continue accelerating. This occurs when the gravitational force and drag become equal. If an object is falling under a source of power other than gravity, such as an engine, then it will take longer for it to reach its terminal velocity.