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]

WordNet (r) 3.0 (2006):

ambassador
    n 1: a diplomat of the highest rank; accredited as
         representative from one country to another [syn:
         ambassador, embassador]
    2: an informal representative; "an ambassador of good will"

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



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).


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

AMBASSADOR, international law. A public minister sent abroad by some
sovereign state or prince, with a legal commission and authority to transact
business on behalf of his country with the government to which he is sent.
He is a minister of the highest rank, and represents the person of his
sovereign.
     2. The United States have always been represented by ministers
plenipotentiary, never having sent a person of the rank of an, ambassador in
the diplomatic sense. 1 Kent's Com. 39, n.
     3. Ambassadors, when acknowledged as such, are exempted, absolutely
from all allegiance, and from all responsibility to the laws. If, however,
they should be so regardless of their duty, and of the object of their
privilege, as to insult or openly to attack the laws of the government,
their functions may be suspended by a refusal to treat with them, or
application can be made to their own sovereign for their recall, or they may
be dismissed, and required to depart within a reasonable time. By fiction of
law, an ambassador is considered as if he were out of the territory of the
foreign power; and it is an implied agreement among nations, that the
ambassador, while he resides in the foreign state, shall be considered as a
member of his own country, and the government he represents has exclusive
cognizance of his conduct, and control of his person. The attendants of the
ambassador are attached to his person, and the effects in his use are under
his protection and privilege, and, generally, equally exempt from foreign
jurisdiction.
     4. Ambassadors are ordinary or extraordinary. The former designation is
exclusively applied to those sent on permanent missions; the latter, to
those employed on particular or extraordinary occasions, or residing at a
foreign court for an indeterminate period. Vattel, Droit des Gens, 1. 4, c.
6, Sec. 70-79.
     5. The act of Congress of April 30th, 1790, s. 25, makes void any writ
or process sued forth or prosecuted against any ambassador authorized and
received by the president of the United States, or any domestic servant of
such ambassador; and the 25th section of the same act, punishes any person
who shall sue forth or prosecute such writ or process, and all attorneys and
soliciters prosecuting or soliciting in such case, and all officers
executing such writ or process, with an imprisonment not exceeding three
years, and a fine at the discretion of the court. The act provides that
citizens or inhabitants of the United States who were indebted when they
went into the service of an ambassador, shall not be protected as to such
debt; and it requires also that the names of such servants shall be
registered in the office of the secretary of state. The 16th section imposes
the like punishment on any person offering violence to the person of an
ambassador or other minister. P Vide 1 Kent, Com. 14, 38, 182; Rutherf.
Inst. b. 2, c. 9; Vatt. b. 4, c. 8, s. 113; 2 Wash. C. C. R. 435; Ayl. Pand.
245; 1 Bl. Com. 253; Bac. Ab. h.t.; 2 Vin. Ab. 286; Grot. lib. 2, c. 8, 1,
3; 1 Whart. Dig. 382; 2 Id. 314; Dig. l. 50, t. 7; Code I. 10, t. 63, l. 4;
Bouv. Inst. Index, h.t.
     6. The British statute 7 Ann, cap. 12; is similar in its provisions; it
extends to the family and servants of an ambassador, as well when they are
the natives of the country in which the ambassador resides, as when they are
foreigners whom he brings with him. (3 Burr. 1776-7) To constitute a
domestic servant within the meaning of the statute, it is not necessary that
the servant should lodge, at night in the house of the ambassador, but it is
necessary to show the nature of the service he renders and the actual
performance of it. 3 Burr. 1731; Cases Temp. Hardw. 5. He must, in fact,
prove that he is bona fide the ambassador's servant. A land waiter at the
custom house is not such, nor entitled to the privilege of the statute. 1
Burr. 401. A trader is not entitled to the protection of the statute. 3
Burr. 1731; Cases Temp. Hardw. 5. A person in debt cannot be taken into an
ambassador's service in order to protect him. 3 Burr. 1677.