escape velocity
    n 1: the minimum velocity needed to escape a gravitational field

escape velocity \es*cape" vel*o"ci*ty\, n. (Physics)
   The minimum velocity at which an object must be moving in
   order for it to overcome the gravitational attraction of a
   massive celestial body, such as the earth or the sun, and
   escape beyond its gravitational field into free space. The
   velocity is calculated as though attained instantaneously at
   the surface of the celestial body, and is pointed directly
   away from its center, and neglecting effects of atmospheric
   friction. Rockets, which accelerate gradually and are moving
   rapidly at a high altitude when their fuel is exhausted or
   their engines shut off, may escape even if moving slightly
   slower at that point than the surface escape velocity.
   Compare orbital velocity.
   [PJC]

   Note: The escape velocity at the surface of the earth is 11.2
         km/sec (25,100 miles per hour), at the moon's surface
         is 2.4 km/sec, and at the sun's surface is 617.7
         km/sec. The escape velocity is calculated as:
         Ve = root2Rg
         where R is the radius of the celestial body and g is
         the acceleration due to the gravitational field at its
         surface. The peculiar chracteristic of a black hole
         is that the escape velocity at its "surface" (called
         its "event horizon") is greater than the speed of
         light. Therefore nothing, not even light, may escape
         from it. --Dict. Sci. Tech.
         [PJC]