Commissioner is in principle the title given to a member of a commission or to an individual who has been given a commission (official charge or authority to do something, the noun's second meaning).
In practice, the title of commissioner has evolved to include a variety of senior officials, often sitting on a specific commission. In particular, commissioner frequently refers to senior police or government officials. A High Commissioner is equivalent to an ambassador, originally between the United Kingdom and the Dominions and now between all Commonwealth states, whether Commonwealth realms, republics, or countries having a monarch other than that of the realms. The title is also sometimes given to senior officials in the private sector; for instance, many North American sports leagues.