abrupt
Happening suddenly, unexpectedly, and without warning or transition.
Das Gespräch fand ein abruptes Ende, als das Telefon klingelte.
The conversation came to an abrupt end when the phone rang.

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Happening suddenly, unexpectedly, and without warning or transition.
Das Gespräch fand ein abruptes Ende, als das Telefon klingelte.
The conversation came to an abrupt end when the phone rang.

Although 'abrupt' is an adjective, it is very often used like an adverb to describe a verb. It answers the question 'How does something happen?' and then means 'suddenly and unexpectedly.'
Der Fahrer bremste abrupt, um einen Unfall zu vermeiden.
The driver braked abruptly to avoid an accident.
You can recognize the exact meaning of 'abrupt' from the context. If it refers to an action or event, it means 'suddenly.' If it refers to communication, it means 'brusque.' In connection with landscapes, it means 'steep.'
Die Musik hörte abrupt auf.
The music stopped abruptly.
In the meaning of 'plötzlich' (suddenly), 'abrupt' is often used with verbs that describe an end or interruption. Typical combinations are 'abrupt enden' (to end abruptly), 'abrupt stoppen' (to stop abruptly), or 'abrupt abbrechen' (to break off abruptly).
Die Verhandlungen wurden abrupt abgebrochen.
The negotiations were broken off abruptly.
When 'abrupt' describes a person's behavior, it has a negative, unfriendly connotation. An 'abrupte Antwort' (abrupt answer) is not only short, but is also perceived as rude or brusque.
Ihre abrupte Reaktion zeigte allen, dass sie verärgert war.
Her abrupt reaction showed everyone that she was annoyed.
suddenly
'plötzlich' and 'abrupt' both describe something happening unexpectedly and without preparation. 'plötzlich' is very common in everyday language and fits almost any event: a noise starts suddenly, someone suddenly stands up, the weather suddenly changes. 'abrupt' more strongly emphasizes that something ends without a transition or changes in a harsh way. It sounds somewhat more formal and often stronger than 'plötzlich'. If you simply want to say that something happens unexpectedly, 'plötzlich' is usually more natural. If you want to show a sharp break or a quick ending, 'abrupt' is more precise.

Das Meeting endete abrupt, weil der Kunde einen wichtigen Anruf bekam.
The meeting ended abruptly because the customer received an important call.
Während des Gesprächs ist plötzlich der Bildschirm ausgegangen.
During the conversation, the screen suddenly turned off.