Synonyms of 'footnote'
An annotation is a note that is added to a text or diagram, often in order to explain it.
He supplied annotations to 15,000 musical works.
A gloss in the margin explains this unfamiliar word.
Thesaurus for footnote from the Collins English Thesaurus
A footnote is a note at the bottom of a page in a book which provides more detailed information about something that is mentioned on that page.