Unification is the process of turning one nation into another, ‘greater’ nation, with the historical example of the Kingdom of Prussia turning into the German Empire in 1871, as the most famous example.
When talking about Unification, there are two distinct types of Unifications - regular Unifications and Major Unifications, the latter of which have some special mechanics associated with them.
A Regular Unification is, plain and simple, a particular country that can be formed by other countries. These may be countries that have existed in the past, such as Poland, came into being after 1836, such as Italy, are alt-history countries that Paradox have deemed plausible or exciting enough to include, such as Arabia, or may even be countries that already exist on the map in 1836 but can be re-formed if they were to break up or be conquered such as the United Provinces of Central America.
Major Unifications, on the other hand, are countries such as Germany and Italy, which historically had strong nationalist movements agitating for their creation from the disparate polities they were divided into.
At the start of the game, these differ little from regular Unifications, but once any country sharing a primary culture with the Unification Country researches the Nationalism Society tech, new mechanics called Unification Candidates and Unification Diplomatic Plays come into effect.
Unification Candidates are, quite simply, countries that aspire to unify the aforementioned disparate polities under a single flag, the clear historical example here being the Kingdom of Prussia’s ambition to unify Germany. In order for a country to be a Unification Candidate, the country must be at least a Major Power.