A Twitter tweet chat is a pre-arranged chat that happens on Twitter through the use of Twitter updates (called tweets) that include a predefined hashtag to link those tweets together in a virtual conversation.
Formal Twitter tweet chats are arranged in advance and occur at a specific time. They may include a formal agenda with a specific leader or "speaker", or they might involve a free flowing discussion between all participants.
The term tweet chat is sometimes used to refer to conversations that happen on Twitter using hashtags that have not been pre-arranged as a formal chat session. The term can also be confused with the third-party Twitter application, TweetChat.com, which helps users separate tweets related to specific chats to make them easier to follow.