The Collaborative International Dictionary of English v.0.48:

Suzerain \Su"ze*rain\, n. [F., formed fr. sus above, L. susum,
   sursum (fr. sub under + versum, p. p. of vertere to turn),
   after the analogy of souverain, E. sovereign. See Sub-, and
   Verse.]
   A superior lord, to whom fealty is due; a feudal lord; a lord
   paramount.
   [1913 Webster]

WordNet (r) 3.0 (2006):

suzerain
    n 1: a state exercising a degree of dominion over a dependent
         state especially in its foreign affairs