The Collaborative International Dictionary of English v.0.48:

Mulatto \Mu*lat"to\, n.; pl. Mulattoes. [Sp. & Pg. mulato,
   masc., mulata, fem., of a mixed breed, fr. mulo mule, L.
   mulus. See Mule.]
   The offspring of a negress by a white man, or of a white
   woman by a negro, -- usually of a brownish yellow complexion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mulatto
    n 1: an offspring of a black and a white parent

The Devil's Dictionary (1881-1906):

MULATTO, n.  A child of two races, ashamed of both.