From sub- +‎ heading.

subheading (plural subheadings)

1. Any of the headings under which each of the main divisions of a subject may be subdivided.
   • 1981 May 9, Mimi Sheraton, “DE GUSTIBUS; WHEN IS FOOD RICH? IT'S ALL A QUESTION OF SEMANTICS”, in The New York Times‎^[1]:

     And puritans give much the same warning in a manner suggesting that though the eating of rich foods is not among the seven deadly sins, it is at least a bona fide subheading under the general category of gluttony.

   • 2019 September 27, Jane Rosenzweig, “The Whistle-Blower Knows How to Write”, in The New York Times‎^[2]:

     Most subheadings don’t do much to enhance a document. The whistle-blower’s subheadings do what the best subheadings do: They structure the complaint and provide a clear outline of what the document contains: […]

2. A heading or caption subordinate to a main headline, heading, or title especially when inserted as a divider between sections (as of a newspaper or periodical article or story or text of a book).

• subhead
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