No rule for this one
No ending or word group predicts the article of Termin. Learn it as a pair and say it out loud: der Termin.
der Termin
It's der Termin. Termin is masculine, plural die Termine.
Ich habe um zehn einen Termin.
I have an appointment at ten.
Ohne Termin geht es nicht.
Without an appointment it doesn't work.
Ich habe um 15 Uhr einen Termin.
I have an appointment at 3.
No ending or word group predicts the article of Termin. Learn it as a pair and say it out loud: der Termin.