The Collaborative International Dictionary of English v.0.48:

Howdah \How"dah\, n. [Ar. hawdaj.]
   A seat or pavilion, generally covered, fastened on the back
   of an elephant, for the rider or riders. [Written also
   houdah.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

howdah
    n 1: a (usually canopied) seat for riding on the back of a camel
         or elephant [syn: howdah, houdah]