A High Commissioner is a person serving in a special executive capacity.
In the Commonwealth of Nations, a High Commissioner is a diplomatic envoy of one Commonwealth Government to another. As many Commonwealth members share the same head of state, Commonwealth diplomatic relations are at a governmental level, and Commonwealth Governments thus do not appoint ambassadors, who are the envoys of one head of state to another. In the place of an embassy is a High Commission, and outside the capital city Commissioners are appointed instead of diplomatic consuls. Nonetheless, since 1947 Commonwealth High Commissioners have enjoyed the same rank and precedence as ambassadors of foreign heads of state.
Historically, in the British Empire High Commissioners were envoys of the Imperial Government appointed to manage protectorates or groups of territories not fully under the sovereignty of the British Crown. Such High Commissioners were also charged with managing diplomatic relations with native rulers and their states, and might have under them several Resident Commissioners attached to each state. In certain places of particular importance, a Commissioner-General would be appointed, who would have control over several High Commissioners and Governors, e.g. the Commissioner-General for South-East Asia had responsibility for Malaya, Singapore and British North Borneo.
In contrast, crown colonies (which were British sovereign territory) would normally be administered by a Governor, while the most significant possessions, confederations and the independent Commonwealth Dominions would be headed by a Governor-General. The Imperial Conference of 1929 established that the Governors-General in the independent Dominions were not the representatives of the Imperial Government but the personal representatives of the Sovereign, and with the constitutional development of the Dominions and their assumption of control over their own external and foreign relations, the Imperial and Dominion Governments began to exchange High Commissioners.
At the United Nations, a High Commissioner serves a commission composed of representatives of various nations. For instance, the U. N. High Commissioner for Human Rights serves the United Nations Human Rights Commission.