What's the noun for subheading? Here's the word you're looking for.
subheading
any of the headings under which each of the main divisions of a subject may be subdivided
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)
“Self also neglects to provide headings or subheads on the grounds that these were coined by sub-editors and were therefore not worthy of inclusion.”
“So with groups of capitals in titling and subheads, or with small capitals in the flow of text, it's important to separate these letters with extra space.”
“Headlines and subheads can be in the bolder Whitney Bold or Rockwell fonts to compliment and contrast with the lighter font weights.”