The Collaborative International Dictionary of English v.0.48:

Trusteeship \Trus*tee"ship\, n.
   The office or duty of a trustee.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trusteeship
    n 1: a dependent country; administered by another country under
         the supervision of the United Nations [syn: trust
         territory, trusteeship]
    2: the position of trustee

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "trusteeship":
   agency, agentship, assignment, authority, authorization, brevet,
   care, charge, commission, commissioning, commitment, consignment,
   cure, delegated authority, delegation, deputation, devolution,
   devolvement, embassy, empowerment, entrusting, entrustment, errand,
   executorship, exequatur, factorship, full power, jurisdiction,
   legation, license, lieutenancy, mandate, mission, office,
   plenipotentiary power, power of attorney, power to act,
   procuration, proxy, purview, regency, regentship, responsibility,
   task, trust, vicarious authority, warrant