Final answer:
Terminal velocity is the maximum velocity an object can reach when falling through a substance such as air. It occurs when the force of air resistance equals the force of gravity, resulting in a constant velocity.
Step-by-step explanation:
In free fall, an object is falling under the influence of gravity with no other forces acting on it except for negligible air resistance. The maximum velocity that an object can reach when falling through a substance such as air is called terminal velocity. Terminal velocity is when the force of air resistance is equal to the force of gravity, resulting in a constant velocity.
When an object is in free fall, it accelerates due to gravity. However, as it falls faster, the air resistance also increases. Eventually, the two forces balance each other out and the object reaches a state of constant velocity. At terminal velocity, the net force on the object is zero, so it does not accelerate anymore.