polygyny

from WordNet (r) 3.0 (2006)

polygyny
    n 1: having more than one wife at a time

from The Collaborative International Dictionary of English v.0.48

Polygyny \Po*lyg"y*ny\, n. [Poly- + Gr. ? woman, wife.]
   The state or practice of having several wives at the same
   time; marriage to several wives. --H. Spenser.
   [1913 Webster]