The Collaborative International Dictionary of English v.0.48:

extraterrestrial \extraterrestrial\
   ([e^]ks`tr[.a]*t[e^]r`r[e^]s"tr[i^]*al), n.
   a hypothetical form of life existing outside the Earth or its
   atmosphere, especially intelligent life on other planets or
   in other solar systems; as, what would you say to an
   extraterrestrial?.

   Syn: extraterrestrial being, alien.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

extraterrestrial \ex`tra*ter*res"tri*al\
   ([e^]ks`tr[.a]*t[e^]r`r[e^]s"tr[i^]*al), adj.
   originating or located or occurring outside Earth or its
   atmosphere; as, there is no clear evidence for
   extraterrestrial life.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

extraterrestrial
    adj 1: originating or located or occurring outside Earth or its
           atmosphere; "is there extraterrestrial life?"
    n 1: a form of life assumed to exist outside the Earth or its
         atmosphere [syn: extraterrestrial being,
         extraterrestrial, alien]