Terrestrial Planet — A terrestrial planet is a planet that is mostly composed of silicate rocks and may or may not have a relatively thin atmosphere.
The term is derived from the Greek word for Earth, so an alternate definition would be those planets that are more Earth-like than not.
Terrestrial planets are very different from gas giants, which may or may not have solid surfaces and are composed mostly of hydrogen and helium in various physical states. Only one terrestrial planet, Earth, is known to have an active hydrosphere.
Our solar system has four terrestrial planets. From closest to furthest from the sun, they are Mercury, Venus, Earth and Mars.
Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle.
The Moon is similar but lacks an iron core. At one time there were probably many more terrestrials, but most have been ejected from the solar system or otherwise destroyed.
NASA is considering a proposed project called the Terrestrial Planet Finder, which will be capable of detecting terrestrial planets in other solar systems.
All currently-known extrasolar planets are extremely large, and are most likely to be gas giants.