Quick Answer: A commissioner is, in principle, a member of a commission or an individual who has been given a commission. In practice, the title of commissioner has evolved to include a variety of senior officials, often sitting on a specific commission. In particular, the 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 sometimes given to senior officials in the private sector; for instance, many North American sports leagues. There is some confusion between commissioners and commissaries because other European languages use the same word for both.