The Collaborative International Dictionary of English v.0.48:

Imperium \Im*pe"ri*um\, n.; pl. Imperia. [L. See Empire.]
   1. Supreme power; absolute dominion; empire.
      [Webster 1913 Suppl.]

   2. (Law) The right to command, which includes the right to
      employ the force of the state to enforce the laws. It is
      one of the principal attributes of the executive power.
      [Webster 1913 Suppl.]