An official is someone who holds an office (function or mandate, regardless whether it carries an actual working space with it) in an organization or government and participates in the exercise of authority (either his own or that of his superior and/or employer, public or legally private).
A government official or functionary is an official who is involved in public administration or government, through either election, appointment, selection, or employment. A bureaucrat is a member of the bureaucracy. An elected official is a person who is an official by virtue of an election. Officials may also be appointed ex officio (by virtue of another office, often in a specified capacity, such as presiding, advisory, secretary). Some official positions may be inherited.
A person who currently holds an office is referred to as an incumbent.
The word official as a noun has been recorded since the Middle English period, first seen in 1314. It comes from the Old French official (12th century), from the Latin officialis ("attendant to a magistrate, public official"), the noun use of the original adjective officialis ("of or belonging to duty, service, or office") from officium ("office"). The meaning "person in charge of some public work or duty" was first recorded in 1555. The adjective is first attested in English in 1533, via the Old French oficial.
The informal term officialese, the jargon of "officialdom", was first recorded in 1884.