"Migration" is the process of moving threads from one Stack Exchange site to another.
Migration is the operation that occurs when a question is deemed more appropriate for one of the other sites in the Stack Exchange network and five users (or one moderator) vote to close the question as "off-topic because ..." and then select "This question belongs on another site in the Stack Exchange network." The question (and all of its answers and comments) are copied to the new site and the original question is closed.
