Marginalia
Etymology
New Latin, the neuter plural of "marginalis," from Latin "margo," edge or border. Coleridge helped popularize the English term for the running commentary readers leave alongside a text.
noun · /mar-jih-NAY-lee-uh/
Notes, scribbles, and drawings made in the margins of a book.
What it names
A distinct kind of writing — the reader’s live argument with a text, recorded in its edges.
Example
Her secondhand copy was worth more for its marginalia — a previous owner’s furious rebuttals — than for the novel itself.
Related idea
Kin to the palimpsest as a record layered onto an existing text; marginalia turns a reader into a second, marginal author.