The movement of content from an old site implementation to a new site implementation. Content migrations occur after a new CMS is implemented, and all the content has to been moved from one system to another.
This term is sometimes conflated with the more general migration, which is more commonly used to refer to the entire process of switching CMS platforms (including the re-design, the implementation, etc.). Specifically, a content migration refers to the movement of existing content objects once a new CMS implementation is available.
A very specific type of migration is a "forklift," in which the content and design are "lifted" up and integrated within a new content management system. While this sounds like a development process, in actuality it involves quite a bit of automated and manual migration.