A technique used to boost a vehicle's velocity
via a planet or sun's gravitational field
. Simply flying close to a planet can give a substantial velocity increase. Flying close to a sun and applying thrust at perihelion
can result in a much greater increase, by virtue of the fact that kinetic energy increases as the square of velocity
. The closer you can get to the sun, the greater a boost you can get, but getting too close means tidal forces
would probably tear your ship apart.
Gravity swingbys have been a staple in written science fiction since as far back as the Sixties (at least), and have actually been used by NASA to send Galileo to Jupiter and the Pioneer and Voyager probes out of our solar system. I have yet to see them used in cinematic "SF", but this doesn't surprise me. I do seem to remember them being used in a couple of Star Trek episodes.
This is also known as a "slingshot maneuver" - thanks, Lord Brawl!