The Collaborative International Dictionary of English v.0.48:

Embassy \Em"bas*sy\, n.; pl. Embassies. [OF. ambass['e]e,
   embasc['e]e, LL. ambasciata, fr. ambasciare for ambactiare to
   go on a mission, fr. L. ambactus vassal, dependent, of Celtic
   or German origin; cf. W. amaeth husbandman, Goth. andbahts
   servant, G. amt office, OHG. ambaht. Cf. Ambassador.]
   1. The public function of an ambassador; the charge or
      business intrusted to an ambassador or to envoys; a public
      message to; foreign court concerning state affairs; hence,
      any solemn message.
      [1913 Webster]

            He sends the angels on embassies with his decrees.
                                                  --Jer. Taylor.
      [1913 Webster]

   2. The person or persons sent as ambassadors or envoys; the
      ambassador and his suite; envoys.
      [1913 Webster]

   3. The residence or office of an ambassador.
      [1913 Webster]

   Note: Sometimes, but rarely, spelled ambassy.
         [1913 Webster]

WordNet (r) 3.0 (2006):

embassy
    n 1: a diplomatic building where ambassadors live or work
    2: an ambassador and his entourage collectively

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "embassy":
   Dymaxion house, White House, adobe house, advice, agency,
   agentship, assignment, authority, authorization, booking office,
   box office, branch, branch office, brevet, building, cabinet, care,
   casa, chambers, chancellery, chancery, charge, cliff dwelling,
   closet, commission, commissioning, commitment, communication,
   communique, consignment, consular service, consulate,
   corporate headquarters, corps diplomatique, country house,
   country seat, cure, dacha, deanery, delegated authority,
   delegation, den, deputation, devolution, devolvement,
   diplomatic mission, diplomatic service, diplomatic staff, dispatch,
   dwelling house, edifice, empowerment, entrusting, entrustment,
   erection, errand, executive office, executorship, exequatur,
   express, fabric, factorship, farm, farmhouse, foreign office,
   foreign service, full power, hall, headquarters, home office,
   house, houseboat, jurisdiction, lake dwelling, legation, letter,
   license, lieutenancy, living machine, lodge, main office, mandate,
   manor house, manse, message, mission, office, parsonage, penthouse,
   plenipotentiary power, pneumatogram, power of attorney,
   power to act, prefabricated house, presidential palace,
   procuration, proxy, purview, ranch house, rectory, regency,
   regentship, release, responsibility, roof, shop, skyscraper,
   sod house, split-level, structure, study, task, telegram,
   ticket office, town house, trust, trusteeship, vicarage,
   vicarious authority, warrant, word