The Collaborative International Dictionary of English v.0.48:

Protectorate \Pro*tect"or*ate\, n. [Cf. F. protectorat.]
   1. Government by a protector; -- applied especially to the
      government of England by Oliver Cromwell.
      [1913 Webster]

   2. The authority assumed by a superior power over an inferior
      or a dependent one, whereby the former protects the latter
      from invasion and shares in the management of its affairs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

protectorate
    n 1: a state or territory partly controlled by (but not a
         possession of) a stronger state but autonomous in internal
         affairs; protectorates are established by treaty [syn:
         protectorate, associated state]