"Migration" is the process of migrating off-topic questions from a 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 vote to close the question as "off-topic because..." and then selected "This question belongs on another site in the Stack Exchange network." The question (and all of its answers) are copied to the new site, and the original question is closed.

Copied from https://meta.stackexchange.com/tags/migration/info

Users with privilege to access moderator tools can see list of migrated questions and some related statistics in the migration tab of moderator tools.