The Collaborative International Dictionary of English v.0.48:

Polyandry \Pol`y*an"dry\, n. [Poly- + Gr. 'anh`r, 'andro`s, man,
   male: cf. F. polyandrie.]
   The possession by a woman of more than one husband at the
   same time; -- contrasted with monandry.
   [1913 Webster]

   Note: In law, this falls under the head of polygamy.
         [1913 Webster]

WordNet (r) 3.0 (2006):

polyandry
    n 1: having more than one husband at a time