Final answer:
Terminal velocity is reached when the drag force on a falling object equals the force of gravity, resulting in zero acceleration.
Step-by-step explanation:
When the drag force on an object falling through the air equals the force of gravity, the object has reached its terminal velocity. Terminal velocity is the constant velocity achieved by a falling object when the weight of the object is balanced by the upward drag force. At this point, there is no net force acting on the object, resulting in zero acceleration.