Both the original and CommonMark specs use the term “horizontal rule” to describe Markdown syntax that produces an HTML
<hr /> tag. However the HTML5 spec makes no mention of horizontal rules. Instead, the term “thematic break” is used to describe the meaning of the
The hr element represents a paragraph-level thematic break, e.g. a scene change in a story, or a transition to another topic within a section of a reference book.
The meaning has changed since HTML4, where
<hr> did represent a horizontal rule:
The HR element causes a horizontal rule to be rendered by visual user agents.
The meaning was likely changed to make
<hr> have semantic rather than presentational meaning, the same way that
<b> were redefined.
To stay consistent with HTML5 it would be worth changing the terminology in the spec to “thematic break”. Also, the semantic meaning is more accurate; websites may style
<hr> elements as images rather than horizontal rules, for example.