Thread hijack
A thread hijack, often referred to simply as a hijack, is the phenomenon of the conversation in a particular thread in the DP forum moving off its original topic, and a new topic taking over the bulk of that thread's discussion.
At DP, thread hijacks are not usually intentionally executed, but are normally a result of someone making a tangential comment as part of a post which sparks enough interest to lead the discussion into that divergent direction. It's that "catching of others' interest" that can turn a thread "drift" into a full-fledged "hijack."
At some varying point, the thread will likely draw the attention of a moderator, who will often pull out her or his "scissors" and split the hijacked thread into two (or more) pieces, giving each major topic of conversation a thread of its own in an appropriate forum.