Entanglement beyond subsystems
  We present a notion of generalized entanglement which goes beyond the conventional definition based on quantum subsystems. This is accomplished by directly defining entanglement as a property of quantum states relative to a distinguished set of observables singled out by Physics. While recovering standard entanglement as a special case, our notion allows for substantially broader generality and flexibility, being applicable, in particular, to situations where existing tools are not directly useful. 
Lorenza Viola, Howard Barnum, Emanuel Knill, Gerardo Ortiz, Rolando   Somma