From an email:
suzerain (soo-zd-rin or -rayn), n. [Law French] 1. Hist.
A Crown tenant; a tenant in capite holding an estate
immediately of the Crown. 2. Int' I law. A nation that
exercises control over another nation's foreign rela-
tions. Also spelled suzereign.
suzerainty (soo-zd-rin-tee or -rayn-tee). 1. Hist. The
power of a feudal overlord to whom fealty is due. See
FEALTY. 2. Int'llaw. The dominion of a nation that
controls the foreign relations of another nation but
allows it autonomy in its domestic affairs.
"At the present time there appears to be no instance of a
relation between states which is described as a suzerainty.
The term was applied to the relation between Great Britain
and the South African Republic, and also to that between
Turkey and Bulgaria from 1878 to 1909, but it seems likely
to disappear from diplomatic terminology." J.L. Brierly, The
Law of Nations 128 (5th ed. 1955).