The Collaborative International Dictionary of English v.0.48:

Tenno \Ten*no"\, n. [Jap. tenn[=o], fr. Chin. t'ien heaven +
   wang king.]
   Lit., King of Heaven; -- a title of the emperor of Japan as
   the head of the Shinto religion.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

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