Annotation is extra information associated with a particular point in a document.

Most commonly this is used for example in draft documents, where another reader has written notes about the quality of a document at a certain point, "in the margin".

The term marginalia is used to refer to annotations made in books.

Annotations about bibliographical sources, labeled annotated bibliographies, give descriptions about how each source is useful to an author in constructing a paper or argument. Creating these blurbs, usually a few sentences long, establishes a summary for and expresses the relevance of each source prior to writing.

In computing, annotations or comments may be added to a source code document either by a compiler or by the programmer. They do not affect the working of the program but gives explanation (for other programmers, or potential readers of the code principally, but also as a reminder for the author), hints or plans for improvement, etc.

In linguistics, morphological, syntactic, semantic, discourse and pragmatic annotations add information about the linguistic form . Other forms of annotation include comments and metadata; these non-transcriptional annotations are also non-linguistic .