An expression used to describe the way a Web page or an HTML page is both "built" and shown or "served" to a user. Pages built "on the fly" usually contain dynamic content (information that changes frequently), Sometimes as much as each time the page is requested by a user and loaded into a browser.
The opposite of dynamic or "on the fly" Web pages are "static" or "flat" pages and files. These files can only be altered by editing the original HTML file. This method (also referred to as hard coding) means the page has to be manually opened in a HTML or text editor and changed in order for the content on the page to change.