Single page app, everything happens on one "page", although the URL might change. Most state changes are shown to the user without going to the server (they'll go to the server for data, but not html/js/css etc)
Round-trip apps are apps that change pages on every get/post/put request. The round trip is the path the application takes from the browser, to the server, to the browser to display every change in state to the user. This usually results in "flicker" as the web browser has to redraw every page.
Round-trip engineering is a different beast.