The Merge is a two-step procedure, as described by the Foundation. The first phase is the previously stated Bellatrix update, which is scheduled to be live on September 6th at 11:34:47 AM UTC. When this goes live, it starts another clock counting down to a TTD (terminal total difficulty) of 58750000000000000000000. TTD is difficult to express in a single sentence, but it is simply a measure of activity on the Ethereum network. When this level is achieved, another upgrade, Paris, is activated, and the Merge is completed.
