A thread, in the context of internet chat forums, is a series of consecutive message postings to a forum.
The words thread and topic are often (mistakenly) used interchangeably. Topics and threads are similar in concept, but not identical.
The word topic refers to the subject matter of a discussion. For example, the discussion of whether or not it is best to pay for a TV license is really only one topic. There exist within the TT chat forum, however, a number of separate threads. Each of these threads discusses the same topic.
It makes life easier for later information retrieval if all discussions about one topic are confined to just one thread. This is the reason why threads are frequently merged by the Moderators. Same-topic threads are not always merged, however. There are times when it is appropriate to have more than one thread discussing the same topic. For more about this see: Thread merging.
The meaning of the word thread is not intuitively obvious to those who are less computer literate. Frequently they will post a message in which they talk about the string. By string they mean, of course, the thread - but they have not fully grasped the standard vocabulary.
See also: