What is Ambassador?

Legal Definition
An ambassador is an official envoy, especially a highest ranking diplomat who represents a state and is usually accredited to another sovereign state, or to an international organization as the resident representative of their own government or sovereign or appointed for a special and often temporary diplomatic assignment. The word is also often used more liberally for persons who are known, without national appointment, to represent certain professions, activities and fields of endeavor. An ambassador is the ranking government representative stationed in a foreign capital. The host country typically allows the ambassador control of specific territory called an embassy, whose territory, staff, and vehicles are generally afforded diplomatic immunity in the host country. Under the Vienna Convention on Diplomatic Relations, an ambassador has the highest diplomatic rank. Countries may choose to maintain diplomatic relations at a lower level by appointing a chargé d'affaires in place of an ambassador.

The equivalent to an Ambassador exchanged among members of the Commonwealth of Nations are known as High Commissioners. The "ambassadors" of the Holy See are known as Papal or Apostolic Nuncios.
-- Wikipedia
Legal Definition
Interaational 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 rald 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, 70-79.

5. The act of dtigress 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 proseeute 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.
-- Bouviers Law Dictionary
Legal Definition
In international law. A public officer, clothed with high diplomatic powers, commissioned by a sovereign prince or state to transact the international business of his government at the court of the country to winch he is sent. Ambassador is the commissioner who represents one country in the seat of government of another. He is a public minister, which, usually, a consul is not. Brown. Ambassador is a person sent by one sever-eign to another, with authority, by letters of credence, to treat on affairs of state. Jacob.
-- Black's Law Dictionary