“Canon – Piece of music where one voice repeats the part of another, throughout the whole piece. ” Source

This is a pretty good definition from this website:

A canon is a piece of music that uses imitative counterpoint (see fugue). So, if a fugue is counterpoint, and canon is counterpoint – whats the difference?

Well, in a fugue, an entire musical theme is stated (or mostly stated) before the next one starts in. In a canon, the first one just gets going when the next one starts. Rounds (like ‘Row, row, row your boat’) are special canons called perpetual canons.

A canon is also a law, a principle, a collection of works, a criterion or standard for assessment or judging. It is a versatile word.

And a pretty good brand of camera.

Row row row your boat, gently down the stream…