Real Dictionary

footnote
countable noun: 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.
countable noun: If you refer to what you are saying as a footnote, you mean that you are adding some information that is related to what has just been mentioned.

footnote in American English
a note of comment or reference at the bottom of a page
to add a footnote or footnotes to
noun: an explanatory or documenting note or comment at the bottom of a page, referring to a specific part of the text on the page
transitive verb: to add a footnote or footnotes to (a text, statement, etc.); annotate

footnote in British English
noun: a note printed at the bottom of a page, to which attention is drawn by means of a reference mark in the body of the text
verb: to supply (a page, book, etc) with footnotes