Aphelion – the point in a planet’s orbit where it is farthest from the Sun.

Axis – imaginary straight line about which a rotating body spins.

Ellipse – a flattened, closed curve; it describes the orbit of all planets around the Dun.

Galaxy – a large group of millions (perhaps billions) of stars, held in place by gravity.

Gravitational Attraction – a force of attraction, which exists between any two objects in the universe.

Inertia – a scientific force which states that moving objects tend to move in a straight line un less acted upon by some other force.

Milky Way – a narrow band of millions of stars, spiraling in space; "houses" our Solar System.

Nebula – an enormous body of gas and dust rotating is space.

Orbit – the path taken by an object as it moves around a second, central object.

Perihelion - the point in a planet’s orbit where it is closest to the Sun.

Planets – large objects composed of rock and/or gas compounds, moving around a central star.

Revolution – moving of a body in a circular path around a central point.

Rotation – turning motion of a body on its axis.

For More Space Terms Link to:

Glossary of Space Flight Terms
http://www.jpl.nasa.gov/basics/glossary.html
This site offers basic space flight terms from the NASA’S Jet Propulsion Laboratory.