
der Treff
meetup
An informal appointment or planned meeting of people. Short form of 'Treffen' (meeting).
Machen wir morgen einen Treff im Café?
Shall we arrange a meetup at the café tomorrow?
meetup / hangout
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meetup
An informal appointment or planned meeting of people. Short form of 'Treffen' (meeting).
Machen wir morgen einen Treff im Café?
Shall we arrange a meetup at the café tomorrow?

hangout
A place where people meet regularly. Short form of 'Treffpunkt' (meeting point).
Das Jugendzentrum ist ein beliebter Treff für die Teenager aus der Nachbarschaft.
The youth center is a popular hangout for the teenagers from the neighborhood.
'Der Treff' is a nominalization derived directly from the verb 'treffen' (to meet). So it describes either the result or the place of meeting.
Wir treffen uns später. Der Treff ist vor dem Kino.
We're meeting later. The meeting point is in front of the cinema.
'Treff' is a colloquial short form for 'das Treffen' (the meeting) or 'der Treffpunkt' (the meeting point). It is used less often in formal or written contexts.
Lass uns einen Treff für nächste Woche ausmachen.
Let's arrange a meetup for next week.
Whether a meeting or a place is meant can be recognized from the verb. 'Einen Treff haben/ausmachen' (to have/arrange a meetup) refers to the appointment. 'Ein Treff sein' (to be a hangout) or prepositions like 'am Treff' (at the meeting point/hangout) refer to the place.
Wir haben einen Treff um 17 Uhr.
We have a meetup at 5 p.m.
When 'Treff' means an appointment or meetup, it is often combined with the verbs 'ausmachen' (to arrange) or 'vereinbaren' (to agree on/arrange).
Wir haben einen Treff im Park ausgemacht.
We arranged a meetup in the park.
to meet / to hit
Treffen and der Treff can both refer to a planned gathering of people. Treffen is the neutral standard term, while der Treff sounds more colloquial and concise. Both are possible in an ordinary conversation, but Treffen is usually better suited to neutral texts.

Unser Treff am Samstag findet wieder im Park statt.
Our meetup on Saturday will be in the park again.
Wir treffen uns am Samstag im Park.
We’re meeting in the park on Saturday.