Glossary Term Definition
A heading or subheading set to the side of the main text column rather than above it.
Definition
A sidehead is a heading placed in the margin or to the side of the text block rather than centered above it or run into the paragraph. Sideheads offer a way to organize content without interrupting the vertical flow of the text, making them especially useful in reference works, technical manuals, and annotated editions. They can be set flush left in the margin, hung into the margin from the text block, or aligned with the outer edge of the column. Effective use of sideheads requires a text block narrow enough to leave adequate marginal space.
Source
No access — Robert Bringhurst — p. 344:
A heading or subhead set in the margin, beside the main text, instead of being placed above or below it.