The Collaborative International Dictionary of English v.0.48:

Pentarchy \Pen"tar*chy\, n. [Gr. ?: cf. F. pentarchie. See
   Penta-, and -archy.]
   A government in the hands of five persons; five joint rulers.
   --P. Fletcher. "The pentarchy of the senses." --A. Brewer.
   [1913 Webster]