I would say the best way to describe might be ‘an aggregator of all chosen sources’, perhaps a Topic List with the Latest combined from n sources.
It’s open to debate what happens when you click on a topic. Perhaps, if the source is a remote forum, you are just redirected to the source forum immediately in a new tab. So you might argue it’s not a forum of forums as you are still deferring to each local forum to interact further.
This all comes down to a more detailed functional specification.