A thread is a thread and shall always be a thread. It denotes 'infrastructure' for want of a better word. So this thread is part of the infrastructure of this node which is part of the infrastructure of the wider forum. With me so far?
A 'topic' from the interwebs:
"A topic is a subject. It's what you're discussing or what a newspaper article is about, the theme of a documentary, or the focus of your term paper. Green energy, the Oscar nominees, what's on the lunch menu, boxers or briefs — these are all considered topics."
The topic may change and evolve as the conversation progresses. So, as an example, the original topic might be, to take from the quote above, Oscar nominees. It will then naturally evolve to Oscar winners, and then the previous movie of the Oscar winner and then the prequel to the previous movie and then the original book where the idea of the movie came from and then the author and his pet dog and then 'I have a dog just like that'. Conversations naturally flow. Topics change. Threads do not.