The Collaborative International Dictionary of English v.0.48:

Favoritism \Fa"vor*it*ism\, n. [Cf. F. favoritisme.]
   The disposition to favor and promote the interest of one
   person or family, or of one class of men, to the neglect of
   others having equal claims; partiality.
   [1913 Webster]

         A spirit of favoritism to the Bank of the United
         States.                                  --A. Hamilton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

favoritism
    n 1: an inclination to favor some person or group [syn:
         favoritism, favouritism]
    2: unfair treatment of a person or group on the basis of
       prejudice [syn: discrimination, favoritism,
       favouritism]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "favoritism":
   bent, bias, discrimination, forejudgment, inclination, inequality,
   interest, involvement, jaundice, jaundiced eye, leaning, nepotism,
   one-sidedness, parti pris, partialism, partiality, partisanism,
   partisanship, preconception, predilection, predisposition,
   preference, preferential treatment, preferment, prejudgment,
   prejudice, prepossession, proclivity, twist, undetachment,
   undispassionateness, unneutrality