The northernmost state of the Federal Republic of Germany, bordering Denmark and occupying the lower part of the Jutland peninsula. The present day state is without North Schleswig or South Jylland, which is Danish nowadays. Historically, Holstein had always been part of the German kingdom and HREGN later on, whereas Schleswig had not. Holstein was and is German settled but Schleswig historical had a more mixed population. Both duchies were united very early on, and made this even a demand to all its rulers (“Op ewig ungedeelt”, forever undivided), and both came into personal union with Denmark very early on. This combination caused some friction between Germany and Denmark in the 19th century.
Carlton Bach hails from Hamburg-Altona, but he considers that part of Schleswig-Holstein, which he calls 'Occupied Denmark'. His justification for that is that Altona, while nowadays continous part of Hamburg, only administratively became part of it as a regional reform initiated under Nazi Germany.
Has a bewildering variety of names in different Germanic languages - which one is used, of course, can be somewhat political as it implies that country's ownership of the region. Consequently, half of every thread dealing with it will be about what to call it. In summary: