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 their own or that of their 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 or civil servant 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.
In Canon law, the word or its Latin original officialis is used absolutely as the legal title of a diocesan bishop's judicial vicar who shares the bishop's ordinary judicial power over the diocese and presides over the diocesan ecclesiastical court.
The 1983 Code of Canon Law gives precedence to the title Judicial Vicar, rather than that of Officialis (canon 1420). The Code of Canons of the Eastern Churches uses only the title Judicial Vicar (canon 191).
In German, the related noun Offizialat was also used for an official bureau in a diocese that did much of its administration, comprising the vicariate-general, an adjoined secretariat, a registry office and a chancery.
In sports, the term official is used to describe a person enforcing playing rules in the capacity of a linesman, referee and umpire; also specified by the discipline, e.g. American football official, Ice hockey official.
The term officer is close to being a synonym (but has more military connotations). A functionary is someone who carries out a particular role within an organization; this again is quite a close synonym for official, as a noun, but with connotations closer to bureaucrat. Any such person acts in their official capacity, in carrying out the duties of their office; they are also said to officiate, for example in a ceremony. A public official is an official of central or local government.
Max Weber gave as definition of a bureaucratic official:
An official must exercise their judgment and their skills, but their duty is to place these at the service of a higher authority; ultimately they are responsible only for the impartial execution of assigned tasks and must sacrifice their personal judgment if it runs counter to their official duties.
As an adjective, "official" often, but not always, means pertaining to the government, as state employee or having state recognition, or analogous to governance or to a formal (especially legally regulated) proceeding as opposed to informal business. Some examples: