General name of the German territory along the Rhine roughly below the mouth of the River Main. The modern borders of the currently defined territory were given by Prussia, of which “Rhineland” was a province. However, presently it is divided between the two German states North-Rhine Westphalia and Rhineland-Palatinate. Historically, a full Rhine border had been a French aim for a very long time. The French did annex the west bank during the Revolutionary Wars, but it fell to Prussia afterwards as mentioned above. After WW1 it was first occupied by the French and then demilitarised, but Hitler restored German military sovereignty there 1936 without repercussions. While there the French tried to get a Rhenish separatist movement started (see flag below). That was wishful thinking on their part, and the whole “movement” was much French hot air, and nearly no German members; although later German Chancellor Adenauer may or may not have been a Rhenish separatist (or for that matter may or may not have been a collobarator with the French occupation troops).
However, somehow the thought of an independent Rhineland stuck, and many believe there was in fact a genuine movement. Consequently, the Rhineland is often independent in many timelines with a disunited Germany.
It is also home to an (in)famous dish called sauerbraten.