| Term | Definition |
| solar system | The Sun and the planets and other objects that orbit the Sun. Also, any star and the objects that revolve around it. |
| geocentric model | A representation of the universe in which stars and planets revolve around Earth. |
| heliocentric model | A representation of the relationship between the Sun and planets in which the planets revolve around the Sun. |
| universe | The sum of everything that exists. |
| big-bang theory | A hypothesis, supported by data, that describes how the universe began with a huge explosion. |
| terrestrial planets | an object in space that resembles Earth in size, in density, and in its mainly rocky composition |
| moon | A natural object that revolves around a planet |
| satellite | A natural or human-built object that revolves around another object in space. |
| gas giants | A large planet that is made up mostly of gaseous and liquid substances, with little or no solid surface. |