Definition - What does Annotation mean?
Annotations are a kind of metadata that makes references to the data itself but is not an actual part of it. Annotations can include notes on metadata, such as comments on the data. Annotations can be made in a special program or as part of a markup language or programming language.
Techopedia explains Annotation
Annotations allow users to add comments to data. They can range from simple sticky notes to bookmarking tools that allow users to add comments to websites they want to save.
Annotations are a part of note-taking programs such as Evernote and other organizing programs. Users can add notes about web pages, text notes and photos they've taken to remember them later. Other more formalized tools are used is areas such as medical imaging to add information to x-rays or other images.