The Collaborative International Dictionary of English v.0.48:

Octoroon \Oc`to*roon"\, n. [L. octo eight + -roon, as in
   quadroon.]
   The offspring of a quadroon and a white person; a mestee.
   [1913 Webster]

WordNet (r) 3.0 (2006):

octoroon
    n 1: an offspring of a quadroon and a white parent; a person who
         is one-eighth black

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "octoroon":
   Cape Colored, Eurasian, cattalo, citrange, cross, crossbreed,
   griffe, half blood, half-bred, half-breed, half-caste, high yellow,
   hinny, hybrid, ladino, liger, mestee, mestiza, mestizo, metis,
   metisse, mixblood, mixed-blood, mongrel, mulatto, mule, mustee,
   plumcot, quadroon, quintroon, sambo, tangelo, tigon, zebrass,
   zebrule