noun
farewell drink; farewell dinner
Refers to a parting drink or meal, often a formal or semi-formal occasion to say goodbye. Can be used for both the cup itself and the event.
転勤する同僚のために別杯を開いた。
We held a farewell dinner for the colleague who is being transferred.
最後の別杯を交わして、彼は旅立った。
After sharing a final farewell drink, he set off on his journey.
送別会 is a more common and general term for a farewell party, while 別杯 specifically emphasizes the act of sharing a drink or a meal in parting.
Compound of 別 (farewell) and 杯 (cup/sakazuki), literally 'farewell cup'. The exact historical origin is uncertain, but it follows a common pattern of naming social rituals after the shared drink.