In the Commonwealth of Nations, a high commissioner is the senior diplomat (generally ranking as an ambassador) in charge of the diplomatic mission of one Commonwealth government to another. Instead of an embassy, the diplomatic mission is generally called a high commission.
In the British Empire (most of which became the Commonwealth), 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, while Crown colonies (British sovereign territories) were normally administered by a governor, and the most significant possessions, large confederations and the self-governing dominions were headed by a governor-general.
For example, when Cyprus came under British administration in 1878 it remained nominally under the suzerainty of the Ottoman Empire. The representative of the British government and head of the administration was titled high commissioner until Cyprus became a Crown colony in 1925, when the incumbent high commissioner became the first governor. Another example were the high commissioners for Palestine.
A high commissioner could also be charged with the last phase of decolonisation, as in the Crown colony of the Seychelles, where the last governor became high commissioner in 1975, when self-rule under the Crown was granted, until 1976, when the archipelago became an independent republic within the Commonwealth.
As diplomatic residents (as diplomatic ranks were codified, this became a lower class than ambassadors and high commissioners) were sometimes appointed to native rulers, high commissioners could likewise be appointed as British agents of indirect rule over native states. Thus high commissioners could be charged with managing diplomatic relations with native rulers and their states (analogous to the resident minister), and might have under them several resident commissioners or similar agents attached to each state.