The Collaborative International Dictionary of English v.0.48:

Mikado \Mi*ka"do\, n. [Jap.]
   The popular designation of the hereditary sovereign of Japan;
   the emperor of Japan.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mikado
    n 1: the emperor of Japan; when regarded as a religious leader
         the emperor is called tenno [syn: mikado, tenno]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "mikado":
   Dalai Lama, Holy Roman Emperor, Inca, Kaiser, ardri, bey, cacique,
   caesar, cham, czar, kaid, khan, negus, padishah, pendragon,
   pharaoh, rig, sachem, sagamore, shah, sheikh, shogun, tenno,
   tycoon