n.1 a person appointed by a commission to perform a specific task, e.g. the head of the London police, a delegate to the General Assembly of the Church of Scotland, etc.
2 a person appointed as a member of a government commission (Charity Commissioner; Civil Service Commissioner).
3 a representative of the supreme authority in a district, department, etc.
Phrases and idioms:
Commissioner for Oaths a solicitor authorized to administer an oath to a person making an affidavit. Lord (or Lord High) Commissioner the representative of the Crown at the General Assembly of the Church of Scotland.
Etymology: ME f. med.L commissionarius (as COMMISSION)