der Eingriff
procedure
A medical treatment on the body, especially a surgical one.
Der Patient erholt sich gut von dem kleinen chirurgischen Eingriff.
The patient is recovering well from the small surgical procedure.

intervention / procedure
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procedure
A medical treatment on the body, especially a surgical one.
Der Patient erholt sich gut von dem kleinen chirurgischen Eingriff.
The patient is recovering well from the small surgical procedure.

The word 'Eingriff' (intervention/procedure) is derived from the verb 'eingreifen' (to intervene). It literally describes the action of 'reaching into' something and thereby changing it, whether it is a body, a right, or a technical system.
Der Arzt erklärte, dass der Eingriff notwendig sei.
The doctor explained that the procedure was necessary.
'Eingriff' (intervention) is very often used with the preposition 'in' followed by the accusative. This construction describes what the action affects.
Naturschützer kritisieren den Eingriff in das Ökosystem des Flusses.
Conservationists criticize the intervention in the river's ecosystem.
In a medical context, 'Eingriff' (procedure) is often combined with adjectives such as 'chirurgisch' (surgical), 'operativ' (operative), 'klein' (small), or 'groß' (major). A typical action is to 'sich einem Eingriff unterziehen' (to undergo a procedure).
Sie muss sich morgen einem kleinen chirurgischen Eingriff unterziehen.
She has to undergo a small surgical procedure tomorrow.
In a legal or political context, typical combinations are 'ein schwerwiegender Eingriff' (a serious intervention), 'ein unzulässiger Eingriff' (an unlawful intervention), or 'ein Eingriff in die Grundrechte' (an intervention in fundamental rights).
Das Gericht prüft, ob das Gesetz einen Eingriff in die Pressefreiheit darstellt.
The court is examining whether the law constitutes an intervention in freedom of the press.
treatment
'die Behandlung' and 'der Eingriff' can both be medical. 'die Behandlung' is the broader term: it can include conversations, medication, exercises, or multiple appointments. 'der Eingriff' is a specific medical procedure on the body, often a minor or major operation. The words can be confused because an 'Eingriff' can be part of a 'Behandlung'. If you are speaking generally about help from a doctor, 'Behandlung' fits. If something is actually being done to the body, 'Eingriff' is more precise.

Nach dem kleinen Eingriff durfte er noch am selben Tag nach Hause gehen.
After the minor procedure, he was allowed to go home the same day.
Die Behandlung hat mehrere Wochen gedauert, aber jetzt geht es ihm besser.
The treatment lasted several weeks, but now he is doing better.