This is the 4669th most frequent German word.
vereinigen
To unite or combine.
Here 'vereinigen' is used reflexively to indicate the groups coming together in unity.
Die verschiedenen Gruppen beschlossen, sich zu vereinigen.
The different groups decided to unite.
In this case, 'vereinigen' is used transitively to indicate combining theories into a single coherent concept.
Die Wissenschaftler versuchten, die Theorien zu vereinigen.
The scientists tried to unify the theories.
Here 'vereinigen' means to bring together or integrate different elements into one entity.