The Collaborative International Dictionary of English v.0.48:

Soapberry tree \Soap"ber`ry tree`\ (Bot.)
   Any tree of the genus Sapindus, esp. Sapindus saponaria,
   the fleshy part of whose fruit is used instead of soap in
   washing linen; -- also called soap tree.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Sapindus saponaria
    n 1: evergreen of tropical America having pulpy fruit containing
         saponin which was used as soap by Native Americans [syn:
         China tree, false dogwood, jaboncillo, chinaberry,
         Sapindus saponaria]