A web app where this is just one HTML file but the JavaScript gives it the feeling of being a whole site (client side template rendering, etc). I think in essense a single page app is one where there is no server side temp laying.
Say / responds with the homepage as per usual and /feed responds with a different html start point, but the same application code.
Using this technique you could create a site that responds to multiple urls with html and at the same time creates a SPA experience using a single js application (say, with backbone for example).