The Collaborative International Dictionary of English v.0.48:

Rambutan \Ram*bu"tan\ (r[a^]m*b[=oo]"t[a^]n), n. [Malay
   ramb[=u]tan, fr. rambut hair of the head.] (Bot.)
   A Malayan fruit produced by the tree Nephelium lappaceum,
   and closely related to the litchi nut. It is bright red, oval
   in shape, covered with coarse hairs (whence the name), and
   contains a pleasant acid pulp. Called also ramboostan.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rambutan
    n 1: Malayan tree bearing spiny red fruit [syn: rambutan,
         rambotan, rambutan tree, Nephelium lappaceum]
    2: pleasantly acid bright red oval Malayan fruit covered with
       soft spines [syn: rambutan, rambotan]