A slingshot maneuver is a spaceflight maneuver which uses a gravitational field to increase the speed an object.
In 2000, Stargate Command attempted to use a slingshot maneuver around Jupiter to correct the course of the X-301 Interceptor. It was unsuccessful because the AIM 120A air-to-air missiles they used didn't have enough thrust. (SG1: "Tangent")
In 2006, during a skirmish with the Wraith in the Pegasus galaxy, the Odyssey performed a slingshot maneuver around a black hole to draw a Hive ship closer to the phenomenon. This had the effect of disabling the Wraith jamming code and allowed the Odyssey to destroy the ship with nuclear weapons. This also accomplished Odyssey's mission, and resulted in the destruction of an Ori ship in the Milky Way. (SG1: "The Pegasus Project")
A slingshot maneuver was used by the Destiny expedition in 2009 when one of the Destiny's shuttles could not match the speed of the Destiny. The shuttle used a nearby planet to speed up and catch up to the Destiny. The opposite of an slingshot maneuver is a aerobraking maneuver. (SGU: "Darkness", "Light")