Notebooks (*.nb, *.ma in earlier versions) are structured interactive documents produced by Mathematica that can contain text, (animated) graphics, sound, calculations, typeset expressions, and user interface elements. They are expressions themselves, and are represented in the front end as windows (e.g. editable notebook windows or dialog windows).

Questions about editing notebook files (i.e. items in an OS) should use the tag .

Relevant documentation:

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