der Abschied
farewell
The act or moment of saying goodbye when leaving a place or a person.
Der Abschied am Bahnhof fiel ihnen sehr schwer.
The farewell at the train station was very hard for them.

farewell / departure
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farewell
The act or moment of saying goodbye when leaving a place or a person.
Der Abschied am Bahnhof fiel ihnen sehr schwer.
The farewell at the train station was very hard for them.

'Abschied' comes from the old verb 'scheiden', which means 'to separate'. The prefix 'ab-' strengthens the idea of separation. So an 'Abschied' is literally the moment of separation.
Der Abschied von seiner Familie fiel ihm nicht leicht.
The farewell from his family was not easy for him.
'Der Abschied' is the noun form of the verb 'sich verabschieden' (to say goodbye). When you say goodbye to someone, this moment is the 'Abschied' (farewell).
Wir müssen uns jetzt leider verabschieden.
Unfortunately, we have to say goodbye now.
To describe the emotions involved, 'Abschied' is often combined with adjectives such as 'schwer' (hard), 'leicht' (easy), 'herzlich' (warm/cordial), or 'tränenreich' (tearful).
Es war ein sehr herzlicher Abschied unter Freunden.
It was a very warm farewell among friends.
The expression 'Abschied nehmen von + dative' (to say goodbye to) is very common. It emphasizes the conscious, active process of saying goodbye, often to something abstract like an idea or habit.
Er musste von dem Gedanken Abschied nehmen, jemals reich zu werden.
He had to say goodbye to the idea of ever becoming rich.
In a professional context, 'Abschied' is often used with the prepositions 'aus' (from) or 'in' (into): 'der Abschied aus dem Amt' (leaving office) or 'der Abschied in den Ruhestand' (retirement).
Die Firma organisierte eine große Feier für seinen Abschied in den Ruhestand.
The company organized a big celebration for his departure into retirement.
separation
'die Trennung' and 'der Abschied' can both mean that people are no longer together or are going away. The difference lies in the focus. 'Abschied' is the moment or deliberate act of saying goodbye. This can be brief, for example at a train station, or emotional, when a phase of life ends. 'Trennung' is more about a lasting separation or being separated, especially in relationships or with things. You can say goodbye during an 'Abschied', but a 'Trennung' often changes the situation in the longer term.

Am Bahnhof fiel ihm der Abschied von seiner Familie schwer.
At the train station, saying goodbye to his family was hard for him.
Nach der Trennung zog sie in eine kleinere Wohnung.
After the separation, she moved into a smaller apartment.