The Collaborative International Dictionary of English v.0.48:

Ondatra \Ondatra\ prop. n.
   A genus of rodents including the muskrat (Ondatra
   zibethica).

   Syn: genus Ondatra.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Muskrat \Musk"rat`\, n.
   1. (Zool.) A North American aquatic fur-bearing rodent
      (Ondatra zibethica formerly Fiber zibethicus). It
      resembles a rat in color and having a long scaly tail, but
      the tail is compressed, the hind feet are webbed, and the
      ears are concealed in the fur. It has scent glands which
      secrete a substance having a strong odor of musk. Called
      also musquash, musk beaver, ondatra, and sometimes
      water rat.
      [1913 Webster +PJC]

   2. (Zool.) The musk shrew.
      [1913 Webster]

   3. (Zool.) The desman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Ondatra zibethica
    n 1: beaver-like aquatic rodent of North America with dark
         glossy brown fur [syn: muskrat, musquash, Ondatra
         zibethica]