The Collaborative International Dictionary of English v.0.48:

Ambassador \Am*bas"sa*dor\, Embassador \Em*bas"sa*dor\, n. [See
   Embassador.]
   1. A minister of the highest rank sent to a foreign court to
      represent there his sovereign or country.
      [1913 Webster]

   Note: Ambassadors are either ordinary [or resident] or
         extraordinary, that is, sent upon some special or
         unusual occasion or errand. --Abbott.
         [1913 Webster]

   2. An official messenger and representative.
      [1913 Webster]

Easton's 1897 Bible Dictionary:

Ambassador
In the Old Testament the Hebrew word _tsir_, meaning "one who
goes on an errand," is rendered thus (Josh. 9:4; Prov. 13:17;
Isa. 18:2; Jer. 49:14; Obad. 1:1). This is also the rendering of
_melits_, meaning "an interpreter," in 2 Chr. 32:31; and of
_malak_, a "messenger," in 2 Chr. 35:21; Isa. 30:4; 33:7; Ezek.
17:15. This is the name used by the apostle as designating those
who are appointed by God to declare his will (2 Cor. 5:20; Eph.
6:20).

  The Hebrews on various occasions and for various purposes had
recourse to the services of ambassadors, e.g., to contract
alliances (Josh. 9:4), to solicit favours (Num. 20:14), to
remonstrate when wrong was done (Judg. 11:12), to condole with a
young king on the death of his father (2 Sam. 10:2), and to
congratulate a king on his accession to the throne (1 Kings
5:1).

  To do injury to an ambassador was to insult the king who sent
him (2 Sam. 10:5).

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "ambassador":
   agent, ambassadress, apostolic delegate, attache, career diplomat,
   chancellor, charge, commercial attache, consul, consul general,
   consular agent, delegate, deputy, diplomat, diplomatic,
   diplomatic agent, diplomatist, emissary, envoy,
   envoy extraordinary, foreign service officer, internuncio, legate,
   messenger, military attache, minister, minister plenipotentiary,
   minister resident, nuncio, plenipotentiary, representative,
   resident, secretary of legation, vice-consul, vice-legate