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]