der Apparat
device
A technical device or machine designed for a specific function.
Mein alter Fotoapparat funktioniert immer noch einwandfrei.
My old camera still works perfectly.

apparatus / device
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device
A technical device or machine designed for a specific function.
Mein alter Fotoapparat funktioniert immer noch einwandfrei.
My old camera still works perfectly.

The meaning of 'Apparat' is almost always dependent on context. Pay attention to the surrounding words: Is it about technology, politics, biology, or a telephone conversation?
Schalten Sie bitte den Apparat aus, bevor Sie den Raum verlassen.
Please turn off the device before leaving the room.
In the technical meaning, 'Apparat' is often the base word in compound nouns that refer to a specific device.
Mein elektrischer Rasierapparat ist kaputt.
My electric razor is broken.
In a political or social context, 'Apparat' is often used with adjectives like 'bürokratisch' (bureaucratic) or as part of words like 'Staatsapparat' (state apparatus) or 'Verwaltungsapparat' (administrative apparatus).
Der riesige Verwaltungsapparat der EU ist schwer zu verstehen.
The huge administrative apparatus of the EU is hard to understand.
In medicine and biology, 'Apparat' refers to a system of organs. Typical terms are 'Bewegungsapparat' (musculoskeletal system) or 'Verdauungsapparat' (digestive system).
Regelmäßiger Sport ist gut für den gesamten Bewegungsapparat.
Regular exercise is good for the entire musculoskeletal system.
Using 'Apparat' to mean 'Telefon' (telephone) is colloquial and is considered somewhat old-fashioned today. However, the fixed expression 'am Apparat bleiben' (to stay on the line) is still very common.
Einen Moment, bitte bleiben Sie am Apparat.
One moment, please stay on the line.
system
'das System' and 'der Apparat' can both mean a larger unit made up of several parts. 'System' is more general and more neutral: many elements are connected and function as a whole. 'Apparat' often sounds more cumbersome or more strongly organized, for example with a large authority, party, or administration. In medicine, 'Apparat' can also mean an organ system. Learners may confuse the words because both describe complex structures. For a neutral description, 'System' usually fits best. 'Apparat' is more suitable when the organization seems large, bureaucratic, or like a fixed machine.

Im großen Apparat der Verwaltung dauert eine einfache Anfrage oft lange.
In the large apparatus of the administration, a simple request often takes a long time.
Das neue System der Firma macht die Planung der Termine einfacher.
The company's new system makes scheduling appointments easier.