Canon
The word canon has several meanings in different areas.
In religion
A canon in one sense is a rule adopted by a Council of the Catholic or Eastern Orthodox churches. From Greek kanon, for rule or measure. See canon law.
It is also a rank in the Christian church; canons are priests who are specifically attached to a cathedral and have responsibility for some aspect of its running.
In another sense, a canon is a list of books accepted by a religion as authoritative or divinely inspired. The term was originally Christian, referring to books declared divinely inspired by the canons of Church councils. The term has however come to be extended to other religions as well with compound scriptures, thus one can speak for instance of the Pali canon in Buddhism. See biblical canon for a discussion of the canon of Christianity.
In one final sense, a canon is a member of religious group or a person holding a position of authority (prelate) in some Christian faiths.
Canon is also the only correct way to paint an icon.
In literature and art
In usually academic, non-religious contexts, the so-called Western canon is often spoken of. This is a body of literature and art recognized as definitive of Western civilization.
In non-academic, non-religious contexts, canon is used to describe works of fiction that represent "official" records of events in particular fictional settings. For example, in Star Trek, the various live-action television series and movies are considered to be canon whereas the Star Trek novels and cartoon series are not. Events which occur in non-canon works of fiction are not considered a part of the continuity of the rest of the series, and can be disregarded freely by authors and readers alike when considering the setting as a whole. The criteria for deciding whether a particular work of fiction is considered to be canonical for a particular setting or not varies, sometimes being dictated by a particular authority (often a copyright or trademark holder) and sometimes being decided by informal consensus among those who care. Harry Potter fans consider only things stated by J. K. Rowling, herself, canon. Therefore the Harry Potter books, Quidditch Through the Ages and Fantastic Beasts and Where to Find Them are considered canon, whereas the films and other material released by Warner Bros. are not.
In music
In music, a canon is a piece of music which contains a melody and one or more strict imitations of this melody played together, usually at staggered intervals. The imitations can be in a transposed, inverted, or retrograde form.
The canon has its origins in Italy and France and was originally called caccia, the latin word for 'chase'. The Old French canon, which meant 'leaned', was taken from the Greek kanon for a rule or law, which eventually came to mean 'an accepted rule' in English.
Special types of canon include the crab canon or can crizans, in which the follower is the exact reversal of the leader in time and pitch, and the sloth canon or proportional canon, in which one voice sings faster than another.
The most popular canons heard today are from the Baroque period, the most famous being Johann Pachelbel's Canon in D.
Canons that use repetitive phrases (i.e. 'Row, row, row your boat') are special canons referred to as perpetual canons.
Canon is also the name of a Japanese company that specialises in imaging and optical products.