der Termin
appointment
A fixed point in time for a meeting, appointment, or event.
Ich habe morgen um 10 Uhr einen Termin beim Zahnarzt.
I have an appointment at the dentist tomorrow at 10 o'clock.

appointment / deadline
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appointment
A fixed point in time for a meeting, appointment, or event.
Ich habe morgen um 10 Uhr einen Termin beim Zahnarzt.
I have an appointment at the dentist tomorrow at 10 o'clock.

You can often tell from the context whether an appointment or a deadline is meant. If 'Termin' (appointment/deadline) is connected with a person or place, e.g., 'beim Arzt' (at the doctor's) or 'mit dem Kunden' (with the client), it is an appointment. If it is about a task, e.g., 'für die Abgabe' (for submission), it is a deadline.
Mein Termin mit dem Anwalt ist um 15 Uhr.
My appointment with the lawyer is at 3 p.m.
For appointments, 'Termin' (appointment) is often used with the verbs 'haben' (to have), 'vereinbaren' (or 'machen') (to arrange/make), 'verschieben' (to postpone), and 'absagen' (to cancel).
Können wir für nächste Woche einen neuen Termin vereinbaren?
Can we arrange a new appointment for next week?
To say whom you have an appointment with or where you have an appointment, you use the prepositions 'bei' + dative (often for doctors, offices) or 'mit' + dative (for people).
Ich habe heute Nachmittag einen Termin bei meiner Friseurin.
I have an appointment with my hairdresser this afternoon.
When 'Termin' means a deadline, typical verbs are 'einhalten' (to meet/keep), 'verpassen' (to miss), or 'setzen' (to set).
Alle Mitarbeiter müssen den Abgabetermin für den Bericht einhalten.
All employees must meet the submission deadline for the report.
time
'die Zeit' and 'der Termin' are both used when planning, but they do not mean the same thing. 'Zeit haben' means that you are free and can do something. 'der Termin' is a fixed point in time for a meeting, a doctor's appointment, or a deadline. So you can have time but not yet have an appointment. There is a particular risk of confusion with arrangements: for free availability, you say 'Ich habe Zeit' (I have time); for a fixed arrangement, you say 'Ich habe einen Termin' (I have an appointment).

Vor dem Termin beim Arzt hat die Chefin mich noch kurz angerufen.
Before the doctor's appointment, my boss gave me a quick call.
Heute Abend habe ich Zeit für einen Spaziergang mit meiner Freundin.
This evening I have time for a walk with my girlfriend.