die Fixierung
fastening
The fastening or attaching of something so that it can no longer move.
Die korrekte Fixierung des Kindersitzes im Auto ist lebenswichtig.
The correct fastening of the child seat in the car is vital.

fixation
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fastening
The fastening or attaching of something so that it can no longer move.
Die korrekte Fixierung des Kindersitzes im Auto ist lebenswichtig.
The correct fastening of the child seat in the car is vital.

The noun 'die Fixierung' (fixation/fastening) is derived directly from the verb 'fixieren' (to fasten, to determine). The ending '-ung' describes the process or result of this action.
Man muss den Schrank gut an der Wand fixieren, damit er nicht umfällt.
You have to fix the cabinet securely to the wall so that it doesn't fall over.
You can recognize the meaning of 'Fixierung' (fixation/fastening) from the context. If it is about technology and objects, fastening is meant. In a psychological context, it refers to an obsession; in medicine, to restraining patients or preserving samples.
Je nach Fachgebiet hat das Wort „Fixierung“ eine andere Bedeutung.
Depending on the field, the word 'Fixierung' has a different meaning.
The meanings in the medical field (restraining patients) and in biology (preserving tissue) are mainly common in technical language. They are rarely used in everyday speech.
Im Laborprotokoll wird die Methode der Fixierung genau beschrieben.
The method of fixation is described precisely in the lab protocol.
In the technical meaning of fastening, the word is often used in connection with safety, for example 'die Fixierung der Ladung' (securing the load) on a truck or 'die Fixierung eines Regals' (fastening a shelf) to the wall.
Die mangelhafte Fixierung der Bauteile führte zum Einsturz.
The inadequate fastening of the components led to the collapse.
When 'Fixierung' (fixation) describes a psychological obsession, the preposition 'auf' with the accusative is almost always used. This structure is: 'Fixierung auf jemanden/etwas' (fixation on someone/something).
Ihre Fixierung auf den Erfolg macht sie unglücklich.
Her fixation on success makes her unhappy.
concentration
'die Konzentration' and 'die Fixierung' can both mean that attention is strongly directed toward one thing. 'Konzentration' is normally neutral or positive: you gather your thoughts and work attentively. 'Fixierung' is stronger in this meaning and often negative. It means that someone is too strongly or almost compulsively focused on a person, idea, or goal. Learners may confuse the words because both talk about strong focus. If the focus is healthy and useful, 'Konzentration' fits. If it seems excessive and is hardly let go of, 'Fixierung' fits.

Seine Fixierung auf den Erfolg macht ihn seit Wochen unruhig.
His fixation on success has been making him restless for weeks.
Mia verliert schnell die Konzentration, wenn neben ihr laut telefoniert wird.
Mia quickly loses concentration when someone is talking loudly on the phone next to her.