Syntactic difference between models is a wide research area with applications in tools for model evolution, model synchronization and version control. On the other hand, semantic difference between models is rarely discussed. We point out to main use cases of semantic difference between models, and then propose a framework for defining well-formed difference operators on model semantics as adjoints of model combinators such as conjunction, disjunction and structural composition. The framework is defined by properties rather than constructively. We instantiate the framework for two rather different modeling languages: feature models and automata specifications. We believe that the algebraic theory of semantic difference will allow to define practical model differencing tools in the future.