German reunification (Deutsche Wiedervereinigung) took place twice after 1945: first in 1957, the Saarland was permitted to join the Federal Republic of Germany, and again on 3 October 1990, when the five re-established states of the German Democratic Republic (GDR / East Germany) joined the Federal Republic of Germany (FRG / West Germany), and Berlin was united into a single city-state. The start of the latter reunification process is by former citizens of Eastern Germany (GDR) commonly referred to as die Wende (The Turning Point.).
After the GDR's first free elections on 18 March 1990, negotiations between the GDR and FRG culminated in a Unification Treaty, whilst negotiations between the GDR and FRG and the four occupying powers produced the so-called "Two Plus Four Treaty" granting full sovereignty to a unified German state, whose two halves had previously still been bound by a number of limitations stemming from its post-WWII-status as an occupied nation.
After 1990, the term "die Wende" became more common. The term generally refers to the events (mostly in Eastern Europe) that led up to the actual reunification; in its usual context, this term loosely translates to "the turning point", without any further meaning. When referring to the events surrounding unification, however, it carries the cultural connotation of the time and the events in the GDR that brought about this "turnaround" in German history.
Germany was officially reunified at 00:00 CET on 3 October 1990, when the five re-established federal states (Bundesländer) of East Germany - Brandenburg, Mecklenburg-Vorpommern, Saxony, Saxony-Anhalt, Thuringia - formally joined the Federal Republic of Germany, along with the city-state Berlin which formally came into being at the same time, created out of the still formally occupied West Berlin and East Berlin, and admitted to the federation. In practice however, West Berlin had already acted as an 11th state for most purposes, so Berlin is generally not included in the list of "New Länder".
The process chosen was one of two options implemented in the West German constitution (Grundgesetz). As these five newly-founded German states formally joined the Federal Republic in accordance with (the then-existing) Article 23, the area in which the constitution served as the constitution was simply extended to include them. The alternative would have been for East Germany to join as a whole along the lines of a formal union between two German states that then would have had to, amongst other things, create a new constitution for the newly established country.
To facilitate this process and to reassure other countries, some changes were made to the "Basic Law" (constitution). Article 146 was amended so that Article 23 of the current constitution could be used for reunification. After the five "New Länder" of East Germany had joined, the constitution was amended again to indicate that all parts of Germany are now unified. Article 23 was rewritten as keeping it could be understood as an invitation to e.g. Austria to join. However, the constitution can be amended again at some future date and it still permits the adoption of another constitution by the German people at some time in the future.
On 14 November 1990, the German government signed a treaty with Poland, finalising Germany's boundaries as permanent along the Oder-Neisse line, and thus, renouncing any claims to Silesia, East Brandenburg, Farther Pomerania, Gdańsk (Danzig), and territories of the former province of East Prussia. The following month, the first all-German free elections since 1932 were held, resulting in an increased majority for the coalition government of Chancellor Helmut Kohl.
To commemorate the day that marks the official unification of the former East and West Germany in 1990, 3rd October has since then been the official German national holiday, the Day of German Unity (Tag der deutschen Einheit). It replaced the previous national holiday held in West Germany on 17th June commemorating the Uprising of 1953 in East Germany and the national holiday on 7th October in the GDR.