Definition of HTML "flow content"

In the content tree that makes up the body of an HTML document, flow content is the content at the higher level, such as paragraphs and lists, that is typically made up of HTML block elements. The phrasing content at the lower level of the content tree is contained within the block-level elements that make up the flow content.

When the expected content of an HTML element is flow content, you can code either flow content elements or phrasing content elements, with some occasional restrictions. On the other hand, flow content elements can only be used where flow content is allowed.

See Flow Content vs. Phrasing Content for more information.