A gravitational slingshot, gravity assist maneuver or swing-by is the use of the relative movement and gravity of a planet to alter the path and speed of a spacecraft, typically in order to save fuel, time, and expense. Gravity assistance can be used to speed up, slow down and/or change the path of a spacecraft. The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft.
- http://www2.jpl.nasa.gov/basics/bsf4-1.php Basics of Space Flight, Sec. 1 Ch. 4, NASA Jet Propulsion Laboratory