'Terminal velocity' is what the average Joe might call 'final speed'. It is simply the point at which the forces acting on a body reach equilibrium, and acceleration stops.
Generally terminal velocity is used in reference to an object falling through the atmosphere. Gravity causes it to accelerate towards the Earth's surface, but air resistance causes drag, which slows it down. When these two forces are in equilibrium, the falling object remains at a constant velocity, the 'terminal velocity'.
The exact speed of the terminal velocity depends on the shape of the falling object (which determines drag) and the density of the atmosphere (which determines resistance). A human without a parachute has a terminal velocity of about 56.08 m/s (184 ft/s), or roughly 200kph (124 mph).
The exact speed of terminal velocity is hard to determine for a few reasons. First, you cannot actually ever reach terminal velocity, as an object approaches its terminal velocity asymptotically; terminal velocity is only ever approximated, although it can be approximated very, very closely. Moreover, when talking about objects moving through the atmosphere, air density increases as you approach the Earth's surface, at the rate of about 1% per 80 meters (262 ft). This means that the terminal velocity decreases as you descend.
Although we usually refer to terminal velocity as the highest speed a person or object reaches as it falls, this is not always the case. A skydiver speeds up to reach terminal velocity, but a meteor entering the Earth's atmosphere would slow down until it reaches terminal velocity (or the Earth's surface, whichever comes first). Terminal velocity is used in all branches of fluid dynamics, and you can refer to terminal velocities of objects moving through liquids or even plasma. The equations for determining terminal velocity can become quite complex once you move out of the Earth's atmosphere.