The Collaborative International Dictionary of English v.0.48:

Emperor \Em"per*or\, n. [OF. empereor, empereour, F. empereur,
   L. imperator, fr. imperare to command; in in + parare to
   prepare, order. See Parade, and cf. Imperative,
   Empress.]
   The sovereign or supreme monarch of an empire; -- a title of
   dignity superior to that of king; as, the emperor of Germany
   or of Austria; the emperor or Czar of Russia.
   [1913 Webster]

   Emperor goose (Zo["o]l.), a large and handsome goose
      (Philacte canagica), found in Alaska.

   Emperor moth (Zo["o]l.), one of several large and beautiful
      bombycid moths, with transparent spots on the wings; as
      the American Cecropia moth (Platysamia cecropia), and
      the European species (Saturnia pavonia).

   Emperor paper. See under Paper.

   Purple emperor (Zo["o]l.), a large, strong British
      butterfly (Apatura iris).
      [1913 Webster]

WordNet (r) 3.0 (2006):

emperor
    n 1: the male ruler of an empire
    2: red table grape of California
    3: large moth of temperate forests of Eurasia having heavily
       scaled transparent wings [syn: emperor, emperor moth,
       Saturnia pavonia]
    4: large richly colored butterfly [syn: emperor butterfly,
       emperor]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "emperor":
   anointed king, chief, chieftain, crowned head, dynast, grand duke,
   high chief, imperator, king, king-emperor, kinglet, lord paramount,
   majesty, monarch, overlord, paramount, petty king, potentate,
   prince, prince consort, royal, royal personage, royalty, ruler,
   sovereign, suzerain, tetrarch



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

EMPEROR, an officer. This word is synonymous with the Latin imperator; they
are both derived from the. verb imperare. Literally, it signifies he who
commands.
     2. Under the Roman republic, the title emperor was the generic name
given to the commanders-in-chief in the armies. But even then the
application of the word was restrained to the successful commander, who was
declared emperor by the acclamations of the army, and was afterwards honored
with the title by a decree of the senate. 3. It, is now used to designate
some sovereign prince who bears this title. Ayl. Pand. tit. 23.