• EXTRATERRESTRIAL (noun). The noun EXTRATERRESTRIAL has 1 sense: 1. a form of life assumed to exist outside the Earth or its atmosphere. Familiarity information: EXTRATERRESTRIAL used as a noun is very rare. • EXTRATERRESTRIAL (adjective). The adjective EXTRATERRESTRIAL has 1 sense: 1. originating or located or occurring outside Earth or its atmosphere. Familiarity information: EXTRATERRESTRIAL used as an adjective is very rare.