fusionieren
to merge
For companies, organizations, or parties to join together to form a single, larger unit.
Die beiden Banken planen, im nächsten Jahr zu fusionieren.
The two banks are planning to merge next year.

to merge / fuse
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to merge
For companies, organizations, or parties to join together to form a single, larger unit.
Die beiden Banken planen, im nächsten Jahr zu fusionieren.
The two banks are planning to merge next year.

The ending '-ieren' shows that this is a verb borrowed from another language, here Latin. Such verbs are almost always regular and weak in German.
Die Firma fusionierte letztes Jahr mit ihrem größten Konkurrenten.
The company merged with its biggest competitor last year.
The verb is often used with the preposition 'mit' (with) to name the partners in the merger. With 'zu' (into), the result of the merger is described.
Die beiden Abteilungen fusionieren zu einem neuen Team.
The two departments are merging into a new team.
You can recognize the meaning of 'fusionieren' (to merge/fuse) from the context. If it is about 'Unternehmen' (companies) or 'Banken' (banks), the business meaning is intended. With 'Stilen' (styles) or 'Musik' (music), it is about art. With 'Atomen' (atoms) or 'Kernen' (nuclei), the physics meaning is intended.
Die beiden Autohersteller haben angekündigt, zu fusionieren.
The two car manufacturers have announced that they intend to merge.
In a business context, it is typically 'Unternehmen' (companies), 'Firmen' (firms), 'Banken' (banks), or also 'Parteien' (parties) that merge with one another.
Es gibt Gerüchte, dass die zwei größten Telekommunikationsfirmen fusionieren wollen.
There are rumors that the two largest telecommunications companies want to merge.
In the figurative sense, abstract things such as 'Stile' (styles), 'Kulturen' (cultures), 'Ideen' (ideas), or 'Musikrichtungen' (music genres) often fuse.
Der Architekt lässt in seinen Bauten moderne und traditionelle Elemente fusionieren.
The architect lets modern and traditional elements fuse in his buildings.
to take over / take on
'übernehmen' and 'fusionieren' can both be used with companies, organizations, or ideas. 'übernehmen' means that one side gets control, leadership, or ownership of something. The other side often does not remain equally strong. 'fusionieren' means that two or more entities join together to form a new or larger unit. In a merger, the focus is therefore on coming together; in a takeover, the focus is on control by one side. They can be especially easy to confuse in business news. If one company buys and runs another, 'übernehmen' is appropriate. If both companies form a new unit together, 'fusionieren' is appropriate.

Die beiden Vereine fusionieren, damit sie gemeinsam mehr Mitglieder erreichen.
The two clubs are merging so that they can reach more members together.
Ein großer Konzern übernimmt die kleine Firma aus der Nachbarstadt.
A large corporation is taking over the small company from the neighboring town.