The Collaborative International Dictionary of English v.0.48:

Soursop \Sour"sop`\, n. (Bot.)
   The large succulent and slightly acid fruit of a small tree
   (Anona muricata) of the West Indies; also, the tree itself.
   It is closely allied to the custard apple.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soursop
    n 1: small tropical American tree bearing large succulent
         slightly acid fruit [syn: soursop, prickly custard
         apple, soursop tree, Annona muricata]
    2: large spiny tropical fruit with tart pulp related to custard
       apples [syn: soursop, guanabana]