single-page web application
A web site that behaves more like an application program in that, instead of clicking a link causing the web browser to load a whole new web page, changes of state are performed by JavaScript running in the web browser fetching new content or data from the web server and using it to update (parts of) the existing page. This is often done via a protocol like AJAX. This way of working allows the browser to maintain the user's session state more easily and minimise the amount of data that needs to be downloaded and rendered thus largely eliminating the delay incurred when moving from page to page in a traditional web site.
Gmail is a well-known example of a single-page web application.