The Collaborative International Dictionary of English v.0.48:

Quadroon \Quad*roon"\, n. [F. quarteron, or Sp. cuarteron. See
   Quarter a fourth part, and cf. Quarteron.]
   The offspring of a mulatto and a white person; a person
   quarter-blooded. [Written also quarteron, quarteroon, and
   quateron.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "quadroon":
   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,
   octoroon, plumcot, quintroon, sambo, tangelo, tigon, zebrass,
   zebrule



Bouvier's Law Dictionary, Revised 6th Ed (1856):

QUADROON. A person who is descended from a white person, and another person
who has an equal mixture of the European and African blood. 2 Bailey, 558.
Vide Mulatto.