A marriage officiant is the person responsible for solemnizing solemnizing marriage. Solemnizing marriage is the legal jargon for conducting a wedding.
A marriage officiant is more commonly referred to as a "Wedding Officiant" though the terms are interchangeable. If a minister, priest, judge, rabi, imam, etc conducts a wedding ceremony, they are acting as a marriage officiant. The term marriage officiant or wedding officiant is more a description of a function than a title. For instance a priest may perform many functions, funerals, services, volunteering, etc. When they are performing marriage they are still a priest performing the function of a marriage officiant.