There exist many kinds of annotation systems and therefore the term is quite ambiguous and context dependent. A general definition was given by Cousins et al. (2000):
Definition of annotation: An annotation is a commentary on an object that:
Wikipedia defines an annotation as “a marked-up comment made to information in a book, document, online record, video, software code or other information.”, retrieved 18:22, 2 July 2009 (UTC).
Used in education, annotation system can refer to a variety of technologies, e.g.: