The Collaborative International Dictionary of English v.0.48:

Directorate \Di*rect"o*rate\, n. [Cf. F. directorat.]
   The office of director; also, a body of directors taken
   jointly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

directorate
    n 1: a group of persons chosen to govern the affairs of a
         corporation or other large institution [syn: directorate,
         board of directors]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "directorate":
   board, board of directors, board of regents, board of trustees,
   bureaucracy, cabinet, cadre, captainship, chairmanship,
   convenership, council, custody, dictatorship, directorship,
   directory, executive arm, executive committee, executive hierarchy,
   foremanship, generalship, governing board, governing body,
   governorship, guardianship, headship, hierarchy, higher echelons,
   higher-ups, infrastructure, intendancy, interlocking directorate,
   leadership, management, managership, mastership, ministry,
   officialdom, overseership, prelacy, presidency, proctorship,
   ruling class, ruling classes, sovereignty, steering committee,
   stewardship, superintendence, superintendency, supervisorship,
   the Establishment, the administration, the authorities, the brass,
   the executive, the ingroup, the interests, the people upstairs,
   the power elite, the power structure, the top, them, they,
   top brass