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.]

WordNet (r) 3.0 (2006):

imperium
    n 1: the domain ruled by an emperor or empress; the region over
         which imperial dominion is exercised [syn: empire,
         imperium]
    2: supreme authority; absolute dominion

Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMPERIUM. The right to command, which includes the right to employ the force
of the state to enforce the laws; this is one of the principal attributes of
the power of the executive. 1 Toull. n. 58.