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?

'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.
appointment / deadline
'der Termin' and 'der Treff' can both refer to a planned get-together of people. However, 'Termin' is more general and neutral. You use it for fixed times, including with doctors, authorities, work, or submissions. 'Treff' is more informal and means either a casual meetup or a place where people meet. The words can be confused because both have to do with meeting. If the exact time and the planning are important, 'Termin' fits. If it is casual, private, or a meeting point, 'Treff' sounds more natural.

In der Stadt war der kleine Platz vor dem Kino unser Treff.
In town, the small square in front of the cinema was our hangout.
Wir haben morgen einen Termin bei der Bank, weil wir Fragen zum Konto haben.
We have an appointment at the bank tomorrow because we have questions about the account.