unerlässlich
indispensable
Absolutely necessary and essential; so important that one cannot do without it.
Gute Sprachkenntnisse sind für diese Position unerlässlich.
Good language skills are indispensable for this position.

indispensable
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indispensable
Absolutely necessary and essential; so important that one cannot do without it.
Gute Sprachkenntnisse sind für diese Position unerlässlich.
Good language skills are indispensable for this position.

The word is made up of the negation 'un-' and 'erlässlich'. 'Erlassen' means to do without something. So 'unerlässlich' (indispensable) describes something that one cannot do without.
Eine gute Vorbereitung ist für den Erfolg des Projekts unerlässlich.
Good preparation is indispensable for the success of the project.
'Unerlässlich' (indispensable) is a formal and strong word. It is often used in written language, in official contexts, or in specialist language to emphasize an absolute necessity.
Die Einhaltung der Sicherheitsvorschriften ist für alle Mitarbeiter unerlässlich.
Compliance with the safety regulations is indispensable for all employees.
'Unerlässlich' (indispensable) is very often used with the preposition 'für' and the accusative to express what something is necessary for. The structure is: Etwas ist unerlässlich für jemanden/etwas (something is indispensable for someone/something).
Regelmäßiges Training ist für eine gute Gesundheit unerlässlich.
Regular exercise is indispensable for good health.
necessary
'nötig' and 'unerlässlich' both mean that something is needed. 'nötig' is neutral and can be used for both small and large things. 'unerlässlich' is much stronger: it means that you really cannot do without something. The word also sounds somewhat more formal. If something is simply needed, 'nötig' fits. If something is absolutely important, 'unerlässlich' is a better fit.

Gute Vorbereitung ist für diese Prüfung unerlässlich.
Good preparation is indispensable for this exam.
Nach dem Unterricht ist noch eine kurze Besprechung nötig.
After class, a short meeting is still necessary.