der Akt
act
A main section of a play, opera, or ballet.
Die dramatischste Szene findet im zweiten Akt statt.
The most dramatic scene takes place in the second act.

act / nude / hassle
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act
A main section of a play, opera, or ballet.
Die dramatischste Szene findet im zweiten Akt statt.
The most dramatic scene takes place in the second act.

'Der Akt' (act) is not a general word for an action like the English 'act'. For a general action, German usually uses 'die Tat' (deed) or 'die Handlung' (action).
Einem alten Menschen über die Straße zu helfen, ist eine gute Tat.
Helping an elderly person cross the street is a good deed.
This meaning is specific to the world of theater, opera, or ballet. When you hear 'erster Akt' (first act) or 'dritter Akt' (third act), it always means a main section of a piece.
Der Vorhang fiel nach dem ersten Akt.
The curtain fell after the first act.
In the meaning of a formal action, 'Akt' (act) is often combined with an adjective or a genitive construction, e.g. 'ein symbolischer Akt' (a symbolic act), 'ein feierlicher Akt' (a solemn act), or 'ein Akt der Verzweiflung' (an act of desperation).
Die Übergabe des Geschenks war ein rein symbolischer Akt.
The handing over of the gift was a purely symbolic act.
When 'Akt' (nude) is used in the context of art, painting, drawing, or photography, it always means the depiction of a naked human body.
Im Museum gibt es eine ganze Abteilung nur für Akte aus dem 19. Jahrhundert.
In the museum, there is an entire section just for nudes from the 19th century.
In colloquial language, 'ein Akt' (a hassle) describes a very complicated or annoying matter. This is often intensified with 'ein riesen Akt' (a huge hassle) or 'ein einziger Akt' (one big hassle).
Ein Visum zu bekommen, war ein riesen Akt mit viel Papierkram.
Getting a visa was a huge hassle with a lot of paperwork.
scene
'die Szene' and 'der Akt' both belong to the language of theater, opera, or ballet. They are easy to confuse because both refer to parts of a work. The difference is in their size: an 'Akt' is a large main section. A 'Szene' is a smaller section within a work, often with a specific action in one place. So in a play, an act can contain several scenes. As a memory aid: the act is the large part, the scene is the smaller part.

Nach dem zweiten Akt gehen die Gäste in die Pause.
After the second act, the guests go on intermission.
In der letzten Szene spricht die Tochter endlich mit ihrem Vater.
In the last scene, the daughter finally speaks with her father.