
der Betrieb
business / company
A business, company, or production facility where people work.
Der Betrieb hat über 500 Mitarbeiter und exportiert in die ganze Welt.
The company has over 500 employees and exports all over the world.
business / operation
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business / company
A business, company, or production facility where people work.
Der Betrieb hat über 500 Mitarbeiter und exportiert in die ganze Welt.
The company has over 500 employees and exports all over the world.

operation
The state in which a machine, facility, or system is functioning and in use; ongoing operation.
Die neue U-Bahn-Linie nimmt nächste Woche den Betrieb auf.
The new subway line will begin operation next week.

activity / bustle
Colloquial for a lot of activity, movement, or noise in a place; lively bustle.
Auf dem Bahnhof war heute Morgen ein unglaublicher Betrieb.
There was an incredible amount of activity at the train station this morning.

out of order
A fixed expression meaning that something, such as a machine or facility, is not working or is temporarily switched off.
Der Fahrstuhl ist leider schon wieder außer Betrieb.
Unfortunately, the elevator is out of order again.
The noun 'der Betrieb' (business / operation) is closely related to the verb 'betreiben' (to operate / to run), which means 'to keep something running' or 'to manage'. This idea of active operation appears in all meanings, from running a company to a machine operating.
Er betreibt ein kleines Café in der Innenstadt.
He runs a small café in the city center.
Fixed expressions are used to describe the start or end of a facility's operation. You say 'etwas in Betrieb nehmen' (to put something into operation) or 'den Betrieb einstellen' (to cease operations).
Die neue Maschine wird morgen in Betrieb genommen.
The new machine will be put into operation tomorrow.
In the meaning 'viel los' (a lot going on) or 'Trubel' (hustle and bustle), 'Betrieb' (activity / bustle) is colloquial. It is used to describe a lively, sometimes chaotic atmosphere.
Am ersten Tag des Schlussverkaufs war in der Stadt ein riesiger Betrieb.
On the first day of the sale, there was a huge amount of activity in the city.
The expression 'außer Betrieb' (out of order) is always used with the verb 'sein' (to be). It describes that a machine or service is temporarily not working.
Die Rolltreppe zum Bahnsteig ist schon wieder außer Betrieb.
The escalator to the platform is out of order again.
company / undertaking
Both das Unternehmen and der Betrieb can refer to an economic organization. Unternehmen tends to refer to the economic entity as a whole, while Betrieb can place more emphasis on the specific workplace or production facility. Unternehmen is suitable for the entire organization; for the unit where the work is carried out, Betrieb is often used.

In diesem Betrieb werden die Möbel für das Unternehmen hergestellt.
The furniture for the company is produced at this facility.
Das Unternehmen besitzt drei Fabriken und verkauft seine Möbel europaweit.
The company owns three factories and sells its furniture throughout Europe.