What are footnotes called when they aren’t in the footer?

Sidenotes is a common term used to denote the notes, trivia or anything interesting noted by the author on the sidelines of pages of his book. http://en.m.wiktionary.org/wiki/sidenote

At least one author, Robert Bringhurst in his book "The Elements of Typographic Style" has talked about the importance of sidenotes and their formatting issues.

Marginalia, see Wikipedia: Marginalia or margin note.

In LaTeX the command to typeset a text in the margins of a page is \marginpar (Footnotes and Margin Notes)

Sidebar is a general term for such marginal notes. According to a wikipedia article,

In publishing, sidebar is a term for information placed adjacent to an article in a printed or Web publication, graphically separate but with contextual connection.

The term has long been used in newspaper and magazine layout. It is now common in Web design, where sidebars originated as advertising space and have evolved to contain information such as quick links to other parts of the site, or links to related materials on other sites. Online sidebars often include small bits of information such as quotes, polls, lists, pictures, site tools, etc.

The terms marginal notes, marginal markings, graffiti, and graffito are used to refer to the numerous small sidebars in italics within Concrete Mathematics by Graham, Knuth, and Patashnik. These terms appear in the half-page rationale for the notes, given in page vii of the book's Preface.

I've always known them as marginalia (mainly due to references to Fermat's marginalia), or simply, marginal notes. Apparently, there are a number of synonyms which are also used. These include:

• apostil (or originally, apostille)
• postil
• gloss