The Collaborative International Dictionary of English v.0.48:

Formosa \Formosa\ prop. n.
   An island off the coast of China, also called Taiwan. It
   was occupied by Japan from 1895 to 1945, when it was returned
   to Chinese sovereignty. After the Communist revolution which
   took over the Chinese mainland in 1949, the Nationalist
   Chinese under Chang Kai-Shek retreated to the island of
   Formosa and established that island as the base of their
   government, being recognized for several years as the de jure
   possessor of the China seat in the United Nations. The
   capital is Taipei. As of 1998, both the Taiwan government and
   the mainland China government recognized Taiwan as properly a
   part of China, but the island is currently ruled as a de
   facto independent nation, though it does not possess a seat
   in the United Nations. The question of when and under what
   circumstances the island will be reunited with the mainland
   government is still unresolved.

   Syn: Taiwan.
        [PJC]

WordNet (r) 3.0 (2006):

Formosa
    n 1: an island in southeastern Asia 100 miles off the coast of
         mainland China in the South China Sea [syn: Taiwan,
         Formosa]