The Collaborative International Dictionary of English v.0.48:

Legation \Le*ga"tion\ (l[-e]*g[=a]"sh[u^]n), n. [L. legatio: cf.
   F. l['e]gation, It. legazione. See Legate.]
   1. The sending forth or commissioning one person to act for
      another. "The Divine legation of Moses." --Bp. Warburton.
      [1913 Webster]

   2. A legate, or envoy, and the persons associated with him in
      his mission; an embassy; or, in stricter usage, a
      diplomatic minister and his suite; a deputation.
      [1913 Webster]

   3. The place of business or official residence of a
      diplomatic minister at a foreign court or seat of
      government.
      [1913 Webster]

   4. A district under the jurisdiction of a legate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

legation
    n 1: the post or office of legate [syn: legation,
         legateship]
    2: a permanent diplomatic mission headed by a minister [syn:
       legation, foreign mission]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "legation":
   agency, agentship, assignment, authority, authorization,
   booking office, box office, branch, branch office, brevet, cabinet,
   care, chambers, chancellery, chancery, charge, closet, commission,
   commissioning, commitment, consignment, consular service,
   consulate, corporate headquarters, corps diplomatique, cure,
   delegated authority, delegation, den, deputation, devolution,
   devolvement, diplomatic mission, diplomatic service,
   diplomatic staff, embassy, empowerment, entrusting, entrustment,
   errand, executive office, executorship, exequatur, factorship,
   foreign office, foreign service, full power, headquarters,
   home office, jurisdiction, license, lieutenancy, main office,
   mandate, mission, office, plenipotentiary power, power of attorney,
   power to act, procuration, proxy, purview, regency, regentship,
   responsibility, shop, study, task, ticket office, trust,
   trusteeship, vicarious authority, warrant



Bouvier's Law Dictionary, Revised 6th Ed (1856):

LEGATION. An embassy; a mission.
     2. All persons attached to a foreign legation, lawfully acknowledged by
the government of this country, whether they are ambassadors, envoys,
ministers, or attaches, are protected by the act of April 30, 1790, 1
Story's L. U. S. 83, from violence, arrest or molestation. 1 Dall. 117; 1 W.
C. C. R. 232; 11 Wheat. 467; 2 W. C. C. Rep. 435; 4 W. C. C. R. 531; 1
Miles, 366; 1 N & M. 217; 1 Bald. 240; Wheat. Int. Law, 167. Vide
Ambassador; Envoy; Minister.