<iframe src="iframe_example.html" width="150" height="150"></iframe>The above example demonstrates usage of the <iframe> element.
The <iframe> element represents a nested browsing context.
In the above example, I am referencing an external HTML document using the src attribute. Therefore, this document could be maintained independently of the current document (it could even be a page on a third party website). The page is presented inside a nested browsing context. If it contained more content than the size of the <iframe> element, it will grow scrollbars (unless you explicitly state for it not to do so).
Alternatively, you could provide content using the srcdoc attribute.
Traditionally the <iframe> element was referred to as an "inline frame", probably because this is the effect that it creates in most situations — that there's a frame embedded in the current page. Typically the <iframe> element is included within the document's content, and therefore, you could say that it's "inline".