Source : Webster's Revised Unabridged Dictionary (1913)

Hydrus \Hy"drus\, n. [L., a water serpent; also, a certain
   constellation, Gr. "y`dros.] (Astron.)
   A constellation of the southern hemisphere, near the south
   pole.

Source : WordNet®

Hydrus
     n : a constellation in the southern hemisphere near the south
         celestial pole