die Gelassenheit
composure
A state of inner calm, balance, and unconcern, especially in stressful or difficult situations.
Trotz des Chaos bewahrte sie eine bewundernswerte Gelassenheit.
Despite the chaos, she maintained an admirable composure.

composure / serenity
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composure
A state of inner calm, balance, and unconcern, especially in stressful or difficult situations.
Trotz des Chaos bewahrte sie eine bewundernswerte Gelassenheit.
Despite the chaos, she maintained an admirable composure.

The noun 'Gelassenheit' (composure / serenity) is derived from the adjective 'gelassen' (calm, relaxed). The ending '-heit' is often used to turn an adjective into a noun that describes the state or quality.
Er ist ein sehr gelassener Mensch.
He is a very calm person.
'Gelassenheit' (composure) is often combined with certain verbs. Typical phrases are 'Gelassenheit bewahren' (to keep one's composure / to stay calm), 'Gelassenheit ausstrahlen' (to radiate calm), or 'etwas mit Gelassenheit nehmen' (to take something calmly).
Versuche, auch in stressigen Momenten deine Gelassenheit zu bewahren.
Try to keep your composure even in stressful moments.
To describe the kind of composure, adjectives such as 'innere' (inner), 'äußere' (outward), 'olympische' (Olympian), or 'bewundernswerte' (admirable) are often used.
Yoga hilft mir, meine innere Gelassenheit zu finden.
Yoga helps me find my inner serenity.
quiet / calm
'die Ruhe' and 'die Gelassenheit' can both describe a relaxed inner state. 'Ruhe' is more general: you can have inner calm, but also Ruhe in the sense of quiet or rest. 'Gelassenheit' means more specifically that someone stays calm and balanced even under stress or when facing problems. Learners may confuse the words because both sound positive and calm. If you want to emphasize that someone does not get upset easily, 'Gelassenheit' fits better.

Trotz der vielen Beschwerden behält Frau Becker ihre Gelassenheit.
Despite the many complaints, Ms. Becker maintains her composure.
Heute fehlt der Lehrerin die Ruhe, weil ständig jemand anruft.
Today the teacher lacks the calm she needs because someone keeps calling.