The Collaborative International Dictionary of English v.0.48:

Typhoon \Ty*phoon"\, n. [Earlier tuffoon, tuffon, Pg. tuf[~a]o,
   Ar. tuf[=a]n a violent storm; probably fr. Gr. tyfw^n,
   tyfw^s, a violent whirlwind, that rushes upward from the
   earth, whirling clouds of dust (cf. Typhus); or perhaps
   from Chin. t'ai-fung a cyclonic wind.]
   A violent whirlwind; specifically, a violent whirlwind
   occurring in the Chinese seas.
   [1913 Webster]

WordNet (r) 3.0 (2006):

typhoon
    n 1: a tropical cyclone occurring in the western Pacific or
         Indian oceans