das Zusammentreffen
meeting
A planned or chance meeting or gathering of two or more people.
Das Zusammentreffen der Staatschefs fand in Genf statt.
The meeting of the heads of state took place in Geneva.

meeting / convergence
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meeting
A planned or chance meeting or gathering of two or more people.
Das Zusammentreffen der Staatschefs fand in Genf statt.
The meeting of the heads of state took place in Geneva.

This noun is formed from the separable verb 'zusammentreffen' (to meet / to coincide). It consists of 'zusammen' (together) and 'treffen' (to meet). Literally, it describes the act of meeting together.
Das Zusammentreffen der alten Schulfreunde war sehr emotional.
The meeting of the old school friends was very emotional.
A very common construction is 'es kommt zu einem Zusammentreffen' (a meeting occurs / there is a convergence). This phrasing emphasizes that the event takes place.
Auf der Konferenz kam es zu einem Zusammentreffen führender Wissenschaftler.
At the conference, there was a meeting of leading scientists.
You can tell from the context whether a meeting of people or the convergence of circumstances is meant. If it is about people, it refers to a meeting. If it is about abstract things like 'Faktoren' (factors) or 'Ereignisse' (events), the second meaning is intended.
Das Zusammentreffen von starkem Regen und Wind führte zu Schäden.
The convergence of heavy rain and wind led to damage.
When it is about people, the word is often combined with adjectives like 'zufällig' (chance), 'geplant' (planned), 'kurz' (brief), or 'offiziell' (official).
Es war nur ein kurzes Zusammentreffen am Bahnhof.
It was only a brief meeting at the train station.
assembly / meeting
'Versammlung' and 'Zusammentreffen' can both mean that several people come together in one place. However, 'Versammlung' sounds more planned and usually more official. There is often a topic, a chairperson, or a goal. 'Zusammentreffen' is more open: it can be planned or accidental and can also involve only two people. The words can be confused because both describe people meeting. When a group deliberately comes together to discuss or decide something, 'Versammlung' fits better. When the meeting itself is what matters, 'Zusammentreffen' fits better.

Das zufällige Zusammentreffen mit ihrem alten Lehrer hat Johanna sehr gefreut.
The chance meeting with her old teacher made Johanna very happy.
Am Montag leitet Frau Becker die Versammlung des Teams.
On Monday, Ms. Becker is chairing the team meeting.