An annotation is extra information associated with a particular point in a document or other piece of information. It can be a note that includes a comment or explanation, and is sometimes presented in the margin of book pages. Annotations can also be used in digital media, such as videos, images, and other forms of media. From a cognitive perspective, annotation has an important role in learning and instruction, as it involves highlighting, naming or labeling, and commenting on aspects of visual representations to help focus learners attention on specific visual aspects. In the digital imaging community, the term annotation is commonly used for visible metadata superimposed on an image without changing the underlying master image, such as sticky notes, virtual laser pointers, circles, arrows, and black-outs. In the medical imaging community, an annotation is often referred to as a region of interest and is encoded in DICOM format.

An annotation can also refer to a note added by way of comment or explanation. Annotations can be used to help readers understand a text better by providing additional context or analysis. Annotated bibliographies, for example, are lists of citations to books, articles, and documents, each followed by a brief descriptive and evaluative paragraph, the annotation, which informs the reader of the relevance, accuracy, and quality of the sources cited.

In summary, an annotation is a note or extra information added to a document or other piece of information, which can be used to provide additional context, analysis, or explanation. It can be presented in the margin of book pages, used in digital media, or added to citations in an annotated bibliography.