A single file application is a web application where all of the assets are delivered in a single static file. This means that it can be downloaded and used offline without losing functionality. It can also be hosted on almost any type of web server.
TiddlyWiki is an unusual single file application because it stores its data within the same file, and is capable of saving changes to itself.
Every single file application must also by definition be a SinglePageApplication, but the converse is not true.