Regardless of what browsers started out being, I'd define any (popular) modern browser as a "generic thick client for running multiple on-line applications".
Yes, a browser is a thick/rich client, it's just not one that runs a single application. There, I said it.
Anyway, that's not quite what Google was trying to say, so here's the link to their content (which is more a defintion for a lay person, as well as a distinction between a browser and search engine etc);
Google Attempts to Explain What a Web Browser Is [VIDEO]