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]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "extraterrestrial":
   Elysian, Olympian, alien, arcane, barbarian, barbaric, barbarous,
   beatific, beatified, blessed, celestial, eerie, esoteric, ethereal,
   exotic, exterior, external, exterrestrial, exterritorial,
   extragalactic, extralateral, extraliminal, extramundane,
   extramural, extraneous, extrapolar, extraprovincial, extrasolar,
   extraterrene, extraterritorial, extratribal, extrinsic, fey,
   foreign, foreign-born, from on high, glorified, heavenly,
   hypernormal, hyperphysical, in glory, intrusive, mysterious,
   numinous, occult, otherworldly, outland, outlandish, outside,
   paradisaic, paradisal, paradisiac, paradisic, preterhuman,
   preternatural, preternormal, pretersensual, psychic, space,
   spiritual, strange, superhuman, supernal, supernatural,
   supernormal, superphysical, supersensible, supersensual,
   supramundane, supranatural, transcendental, transmundane, ulterior,
   unearthly, unhuman, unworldly