
die Verwendung
use
The use or application of something for a specific purpose.
Die Verwendung von Smartphones ist während der Prüfung verboten.
The use of smartphones is prohibited during the exam.
use
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use
The use or application of something for a specific purpose.
Die Verwendung von Smartphones ist während der Prüfung verboten.
The use of smartphones is prohibited during the exam.

to be used
A fixed expression meaning that something is used or applied.
Diese alte Technik findet heute nur noch selten Verwendung.
This old technique is only rarely used today.
'Die Verwendung' (use) is the nominalization of the verb 'verwenden' (to use). The ending '-ung' forms feminine nouns from verbs that describe the process or result of the action.
Wir müssen das Material sparsam verwenden.
We have to use the material sparingly.
To say what something is used for, you can use the construction 'Verwendung für etwas haben' (to have a use for something). To say what is being used, the phrase is 'die Verwendung von etwas' (the use of something).
Hast du eine Verwendung für diese alten Gläser?
Do you have a use for these old glasses?
The expression 'Verwendung finden' (to be used) is a more formal way of saying that something 'verwendet wird' (is used). It is often used in written or official contexts.
Dieses Verfahren findet in der modernen Medizin keine Verwendung mehr.
This procedure is no longer used in modern medicine.
operation / effort / use
Einsatz and Verwendung both refer to using something. Einsatz tends to emphasize actively deploying a means for a specific task, while Verwendung neutrally describes its use or purpose. If the specific act of using it is the focus, Einsatz is appropriate; for a factual statement about its purpose, use Verwendung.

Die Verwendung einer Wärmebildkamera erleichtert die Suche bei Dunkelheit.
Using a thermal imaging camera makes searching in the dark easier.
Bei der Suche kam eine Wärmebildkamera zum Einsatz.
A thermal imaging camera was used during the search.