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.
He sends the angels on embassies with his decrees.
2. The person or persons sent as ambassadors or envoys; the
ambassador and his suite; envoys.
3. The residence or office of an ambassador.
Note: Sometimes, but rarely, spelled ambassy.