I first encountered the term cross domain security (or cross domain systems, or cross domain solutions, or just CDS) at a workshop in the late 1990s. We were discussing the problem of how to share information with coalition forces even though different countries had different, treaty-based access to US defense information. Even worse, there were coalitions that contained countries who were not on the best of terms (like Japan and Korea).
These days the term has often replaced MLS in the defense community. Some argue that the term has changed in hopes that the community can lower the assurance requirements, thus putting information at risk. Time will tell if this is indeed true.