Gegenschein

Gegenschein is a phenomenon that can be seen in places with very dark skies. A faint spot of light appears on the ecliptic at a point directly opposite the Sun's current position, caused by sunlight reflecting from dust in space.

Roche limit

This exotic-sounding term refers to the closest distance an orbiting body (held together only by its own gravity) can get to its parent planet before being torn apart by tidal forces. The Roche Limit is approximately 2.45 times the radius of the parent body. Also known as the tidal stability limit, but I think we can get more mystery and alternative meanings out of the name given it by the French astronomer Édouard Roche.

Swingby

Swingby is the colloquial term for 'gravitational/gravity assist manoeuvre' or 'sling-shot'. The basic principle is to use the relative motion of a massive body (a planet usually) to steal some momentum in order to accelerate or decelerate a spacecraft. This sounds like cheating, but it really works. The velocity relative to the Sun can be changed, but the speed relative to the swingby planet stays the same. Optimising swingby plans is a fun challenge in astrodynamics. 

You can read up on Gravity Assists in wikipedia, or elsewhere, including many excellent books.

As a real example of a swingby, check out this page for a good description and some vector diagrams of the Nozomi Earth Swingby manoeuvre. However the really classic example of this comes from the Voyager 2 Grand Tour:

Spacecraft speed with spikes at each planet caused by swingby
(note how the Jupiter swingby gave Voyager2 enough speed to escape the gravity of the Sun!)