The Collaborative International Dictionary of English v.0.48:

Gecko \Geck"o\ (g[e^]k"[-o]), n.; pl. Geckoes (g[e^]k"[=o]z).
   [Cf. F. & G. gecko; -- so called from the sound which the
   animal utters.] (Zool.)
   Any lizard of the family Geckonid[ae]. The geckoes are
   small, carnivorous, mostly nocturnal animals with large eyes
   and vertical, elliptical pupils. Their toes are generally
   expanded, and furnished with adhesive disks, by which they
   can run over walls and ceilings. They are numerous in warm
   countries, and a few species are found in Europe and the
   United States. See Wall gecko, Fanfoot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gecko
    n 1: any of various small chiefly tropical and usually nocturnal
         insectivorous terrestrial lizards typically with immovable
         eyelids; completely harmless