(spaceflight) a trajectory that passes close to a planetary body in order to gain energy from its gravitational field WordNet 3.0 & Open English WordNet

A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby which makes use of the relative movement and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense. Wikipedia

A spaceflight maneuver Wikipedia Disambiguation

Space-navigation technique Wikidata

The favorable alteration of the speed and trajectory of a spacecraft as a result of its passing near to a celestial body. Wiktionary