The Collaborative International Dictionary of English v.0.48:

Monopetalous \Mon`o*pet"al*ous\, a. [Mono- + petal: cf. F.
   monop['e]tale.] (Bot.)
   Having only one petal, or the corolla in one piece, or
   composed of petals cohering so as to form a tube or bowl;
   gamopetalous.
   [1913 Webster]

   Note: The most recent authors restrict this form to flowers
         having a solitary petal, as in species of Amorpha,
         and use gamopetalous for a corolla of several petals
         combined into one piece. See Illust. of Gamopetalous.
         [1913 Webster]