Extraterrestrial means coming from beyond Earth, or from somewhere outside our planet. The word breaks down into extra (meaning “outside” or “beyond”) and terrestrial (meaning “of the Earth”). So an extraterrestrial creature would be a living being from another planet or somewhere else in space.

When scientists search for extraterrestrial life, they're looking for any form of life, whether microscopic bacteria on Mars or intelligent beings on distant worlds, that didn't originate on Earth. An extraterrestrial signal would be a radio wave or message coming from space that might have been created by intelligent life elsewhere in the universe.

The word is often shortened to ET (like in a famous movie), and it can work as both an adjective and a noun. You might describe an extraterrestrial spacecraft (adjective), or you might talk about meeting an extraterrestrial (noun).