The Collaborative International Dictionary of English v.0.48:

Polygamia \Pol`y*ga"mi*a\, n. pl. [NL. See Polygamous.] (Bot.)
   (a) A Linn[ae]an class of plants, characterized by having
       both hermaphrodite and unisexual flowers on the same
       plant.
   (b) A name given by Linn[ae]us to file orders of plants
       having syngenesious flowers.
       [1913 Webster]