The Collaborative International Dictionary of English v.0.48:

Kaiser \Kai"ser\, n. [Gr., fr. L. Caesar. Cf. Kesar, and
   Czar.]
   The ancient title of emperors of Germany assumed by King
   William of Prussia when crowned sovereign of the new German
   empire in 1871.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Kaiser
    n 1: the title of the Holy Roman Emperors or the emperors of
         Austria or of Germany until 1918

Moby Thesaurus II by Grady Ward, 1.0:

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