The Collaborative International Dictionary of English v.0.48:

Imperator \Im`pe*ra"tor\, n. [L. See Emperor.] (Rom. Antiq.)
   A commander; a leader; an emperor; -- originally an
   appellation of honor by which Roman soldiers saluted their
   general after an important victory. Subsequently the title
   was conferred as a recognition of great military achievements
   by the senate, whence it carried with it some special
   privileges. After the downfall of the Republic it was assumed
   by Augustus and his successors, and came to have the meaning
   now attached to the word emperor.
   [1913 Webster]