The Collaborative International Dictionary of English v.0.48:

Castoreum \Cas*to"re*um\, n. [L. See Castor.]
   A peculiar bitter orange-brown substance, with strong,
   penetrating odor, found in two sacs between the anus and
   external genitals of the beaver; castor; -- used in medicine
   as an antispasmodic, and by perfumers.
   [1913 Webster]