der Gebrauch
use
The act of using or applying something; the use or utilization of something.
Der Gebrauch von Mobiltelefonen ist während der Prüfung verboten.
The use of mobile phones is prohibited during the exam.

use / custom
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use
The act of using or applying something; the use or utilization of something.
Der Gebrauch von Mobiltelefonen ist während der Prüfung verboten.
The use of mobile phones is prohibited during the exam.

The noun 'der Gebrauch' (use) belongs to the word family of the verb 'gebrauchen' (to use). Both words have the root 'brauchen' (to need), which shows the close connection to the idea of use and necessity.
Der richtige Gebrauch dieses Werkzeugs erfordert etwas Übung.
The correct use of this tool requires some practice.
The word 'Gebrauch' (use) is often used in formal, written, or technical contexts, for example in instructions, contracts, or official notices.
Der Gebrauch von Hilfsmitteln ist in der Prüfung nicht gestattet.
The use of aids is not permitted in the exam.
A very common phrase is the instruction 'vor Gebrauch schütteln' (shake before use), which appears on many liquid products. 'Für den täglichen Gebrauch' (for daily use) is also a common expression.
Diese Salbe ist nur für den äußerlichen Gebrauch bestimmt.
This ointment is intended for external use only.
The phrase 'von etwas Gebrauch machen' (to make use of something) is a formal way to say that you are using an opportunity. It is always formed with the preposition 'von' (of/from) and the dative.
Bitte machen Sie von der Möglichkeit Gebrauch, Fragen zu stellen.
Please make use of the opportunity to ask questions.
When 'Gebrauch' (custom) means a custom or tradition, it often appears with adjectives like 'alt' (old), 'lokal' (local), or 'traditionell' (traditional). Without such clues, the meaning is usually 'Verwendung' (use).
Das Schenken von Brot und Salz beim Einzug ist ein schöner alter Gebrauch.
Giving bread and salt when someone moves in is a lovely old custom.
operation / effort / use
'der Einsatz' and 'der Gebrauch' can both mean that something is used or applied. 'Einsatz' fits especially when something is used deliberately for a task or a specific result, for example a tool, a method, or technology. 'Gebrauch' is more general and simply describes the use of something. It often appears in instructions or fixed expressions like 'beim Gebrauch' (when using / during use). The words can be confused because both have to do with use. Remember: 'Einsatz' sounds purposeful; 'Gebrauch' sounds more general or practical.

Nach dem Gebrauch sollte er das Gerät wieder ausschalten.
After use, he should switch the device off again.
Der Einsatz eines Übersetzungsprogramms hilft beim ersten Lesen des Textes.
The use of a translation program helps with the first reading of the text.