The Collaborative International Dictionary of English v.0.48:

Mangosteen \Man"go*steen\, Mangostan \Man"go*stan\, mangosteen
tree \mangosteen tree\n. [Malay mangusta, mangis.] (Bot.)
   A tree of the East Indies of the genus Garcinia (Garcinia
   Mangostana) with thick leathery leaves. The tree grows to
   the height of eighteen feet, and bears fruit also called
   mangosteen, of the size of a small apple, the pulp of which
   is very delicious food.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mangosteen
    n 1: East Indian tree with thick leathery leaves and edible
         fruit [syn: mangosteen, mangosteen tree, Garcinia
         mangostana]
    2: two- to three-inch tropical fruit with juicy flesh suggestive
       of both peaches and pineapples