It's the speed of a free falling object. The downward force of gravity meets the upward force of drag, causing for zero acceleration.

1) "Terminal velocity, (a) the final speed (usually an impact speed) achieved by an object at the end of its path (where gravity, applied physical acceleration and dragging effects may all play a part), and (b) in skydiving, the maximum speed achievable for a given object's atmospheric freefall, at which deceleration due to atmospheric friction counteracts further acceleration due to gravity, and no further acceleration occurs - this is now the more common usage." Source and further information: http://en.wikipedia.org/wiki/Terminal_velocity_%28disambiguation%29 2) "A free falling object achieves its terminal velocity when the downward force of gravity (Fg)equals the upward force of drag (Fd). This causes the net force on the object to be zero, resulting in an acceleration of zero. Mathematically an object asymptotically approaches and can never reach its terminal velocity." Source and further information: http://en.wikipedia.org/wiki/Terminal_velocity Further information: http://www.grc.nasa.gov/WWW/K-12/airplane/termv.html 3) "Sci-Tech Dictionary: terminal velocity (fluid mechanics) The velocity with which a body moves relative to a fluid when the resultant force acting on it (due to friction, gravity, and so forth) is zero. (physics) The maximum velocity attainable, especially by a freely falling body, under given conditions. Also known as terminal speed." "Sports Science and Medicine: terminal velocity A constant velocity reached by an object falling through a resisting medium and being acted upon by a constant force. A body falling through the atmosphere under the force of gravity accelerates until its terminal velocity is reached. It is the velocity at which the upward drag force is equal to the weight of the falling object. The terminal velocity varies with the weight and orientation of a falling body. An ability to vary terminal velocity is very important in sky-diving." "Military Dictionary: terminal velocity (DOD, NATO) 1. Hypothetical maximum speed a body could attain along a specified flight path under given conditions of weight and thrust if diving through an unlimited distance in air of specified uniform density. 2. Remaining speed of a projectile at the point in its downward path where it is level with the muzzle of the weapon." Source and further information: http://www.answers.com/topic/terminal-velocity-2?cat=technology