Extraterrestrial is an adjective, meaning not of this Earth.

Extraterrestrial rock is rock that did not originate from Earth.

The term extraterrestrial can be adjective or noun. In adjective case, the term extraterrestrial refer occurring or existing beyond the earth's atmosphere. In noun case, the term extraterrestrial refer being from the beyond the earth's atmosphere.

The old word for earth (in latin) is Terra. Terrestrial means "of the earth". When something is called "extraterrestrial" it means something that isn't from the earth.