The Collaborative International Dictionary of English v.0.48:

Fetus \Fe"tus\ (f[=e]"t[u^]s), n.; pl. Fetuses
   (f[=e]"t[u^]s*[e^]z). [L. fetus, foetus, a bringing forth,
   brood, offspring, young ones, cf. fetus fruitful, fructified,
   that is or was filled with young; akin to E. fawn a deer,
   fecundity, felicity, feminine, female, and prob. to do, or
   according to others, to be.]
   The young or embryo of a vertebrate animal in the womb, or in
   the egg; often restricted to the later stages in the
   development of viviparous and oviparous animals. showing the
   main recognizable features of the mature animal, embryo
   being applied to the earlier stages. [Written also
   f[oe]tus.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

fetus
    n 1: an unborn or unhatched vertebrate in the later stages of
         development showing the main recognizable features of the
         mature animal [syn: fetus, foetus]