- unification as Holy Roman Empire in middle ages, and later as Holy Roman Empire of the German Nation, since 800 AD (coronation of Charlemagne, king of franks),
- unification as Confederation of the Rhine in 1806, and later as German Confederation,
- North German Confederation, that later in 1871 became the German Empire
- in some part as Anschluss, incorporation of Republic of Austria to German 3. Reich in 1938,
- unification of Federal Republic of Germany, German Democratic Republic, and the area of West Berlin to modern-day state of Germany, since 1990.
For avoiding misunderstanding which German unification you mean, tag your question with appropriate time-defining tags, like medieval, 19th-century etc. For the process of creation of the Holy Roman Empire consider adding the holy-roman-empire tag.
Questions related to 2. Reich (1871-1918) should be tagged with german-empire tag.
Questions related to West Berlin can also be tagged with the berlin-wall tag.