imperator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An army commander in the Roman Republic.
• noun The supreme power of the Roman emperor.
• noun The head of state and supreme commander in the Roman Empire, in whose name all victories were won.

from The Century Dictionary.

• noun In Roman history:
• noun In general, a commander, chief, or ruler: in this sense a descriptive title (placed after the name) of any one possessing the imperium or power of enforcing his authority, as a general, or a consul, proconsul, or other magistrate.
• noun In later times, more especially, a general-in-chief or holder of an independent command during active service: a title often conferred by the senate on a victorious general, or acclaimed by his army.
• noun After the fall of the republic, the official title (used as a prenomen) of the monarch or supreme ruler as permanent generalissimo of the Roman armies; emperor: originally conferred by the senate for a term, and afterward assumed in perpetuity.
• noun [capitalized] In zoology, a genus of trochiform prosobranchiate gastropods, of the family Turbinidæ. Montfort.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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 wiht 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.