A canopy, or seat fixed on the back of an elephant.
“Leading the array, three stately elephants marched, bearing the Woons in gilded howdahs under gold umbrellas.” —J. W. Palmer: Up and Down the Irrawaddi, chap. xx. p. 169.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Howdah from Fact Monster: