Topic is perfectly appropriate in ways that thread never can be:
- A topic describes content. As others have already pointed out, a thread describes the mode of operation where messages are connected in threads.
- Thread is also a literary term but a topic describes focus at a higher level than a thread which, in literary terms, always runs through a larger body of text.
- A thread is interior to a topic. One topic can have many threads. New topics become new threads.
- A topic is more focussed than a thread. Topics are major parts whereas threads are minor parts in the main discussion.
- Technically, you do not have a thread until there is at least a message in response/reply.
- Likewise, in computer systems threads don’t normally exist independently. They normally execute from a process. However, there are exceptions which is another reason for avoiding the technical computing term where threads are concerned.