The Collaborative International Dictionary of English v.0.48:

Gerbil \Ger"bil\ (j[~e]r"b[i^]l), Gerbille \Ger`bille"\
   (zh[asl]r`b[-e]l"), n. [F. gerbille. Cf. Jerboa.]
   1. (Zool.) One of several species of small, jumping, murine
      burrowing rodents, of the genus Gerbillus and related
      genera of the subfamily Gerbillinae. They have long soft
      pale fur and hind legs adapted for leaping. In their
      leaping powers they resemble the jerboa. They inhabit
      Africa, India, and Southern Europe.
      [1913 Webster + WordNet 1.5]

   2. (Zool.) A rodent (Meriones unguiculatus) of the
      subfamily Gerbillinae that is commonly kept as a pet; it
      is also called the tamarisk gerbil, sand rat and
      jird. Its natural habitats are the dry regions of
      Northern Africa and Asia.
      [PJC]

WordNet (r) 3.0 (2006):

gerbil
    n 1: small Old World burrowing desert rodent with long soft pale
         fur and hind legs adapted for leaping [syn: gerbil,
         gerbille]