one act (of a play)
Used for counting acts in a play or drama.
この芝居は一幕が長い。
The first act of this play is long.
Figurative use: a striking or memorable scene or event, often in a larger story or life experience.
彼の人生の一幕を描いた小説。
A novel depicting a scene from his life.
There was a touching scene at the sports day.
He ended up making a scene arguing at the station.
A heartwarming scene unfolded at the venue.
出来事 is a neutral term for 'event' or 'happening', whereas 一幕 implies a vivid, scene-like episode worth recounting.