A sky hook is a long, very strong, cable in orbit around a planet which rotates around its center of mass in such a way that when one end is closest to the ground, its relative velocity is almost zero. It would function as a kind of space elevator; shuttle craft would anchor to the end and then be lifted into orbit where they would be released. It is closely related to the idea of a beanstalk. The concept was originally described by Y. Artsutanov in 1969. The name was probably coined by Hans Moravec.1
1. Moravec, Hans. "A Non-Synchronous Orbital Skyhook." Journal of the Astronautical Sciences, Vol. 25, No. 4, pp 307-322, 1977.