The Collaborative International Dictionary of English v.0.48:

Guano \Gua"no\, n.; pl. Guanos. [Sp. guano, fr. Peruv. huanu
   dung.]
   A substance found in great abundance on some coasts or
   islands frequented by sea fowls, and composed chiefly of
   their excrement. It is rich in phosphates and ammonia, and is
   used as a powerful fertilizer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

guano
    n 1: the excrement of sea birds; used as fertilizer