[pas]

I guess it is because you migrate data between application versions. And sometimes this means simply applying an SQL script file, sometimes you also combine this with upgrading the DB server itself, then you might also need to do the whole export-import dance (async replication + export-upgrade-import on a secondary instance, then promote that instance and either drop the old and create a new secondary, or do the same exp-imp on the old-primary).

Good question though, it could be "schema upgrade", but sometimes there's a downgrade, so somehow people started calling it migration.

And it's especially important to note, that users don't care about the schema, operators care a bit, developers slightly more, and if there is a dedicated DBA in the loop, now that person might finally really care about it, but they rarely write the migration scripts. And developers care about data. (At least that's my impression.)