I'm trying to preserve the meaning of the English word 'migration' in the definition, and I think I might be able to do that without making up a fake definition. Here goes:
A schema migration is the change of a database's schema from one state to another through time, as is distinct from the more general definition of data migration, in which data is moved from one location to another.
Anyway, this is basically original research, because it's hard to find these kinds of definitions sometimes. So I'd really appreciate some expert feedback.
