The Collaborative International Dictionary of English v.0.48:

Plebiscitum \Ple`bis*ci"tum\, n. [L., fr. plebs, plebis, common
   people + scitum decree.] (Rom. Antiq.)
   A law enacted by the common people, under the superintendence
   of a tribune or some subordinate plebeian magistrate, without
   the intervention of the senate.
   [1913 Webster]