Adieu is a French word for goodbye that English borrowed long ago. It is not an ordinary goodbye. It carries the feeling of a serious parting — one that might last a long time, or forever — and it sounds either literary or playful, depending on who says it and how.

You will meet it in three places. In old books and poetry, where it is sincere. In the fixed phrase 'without further adieu' — though careful writers say that phrase is really 'without further ado', a different word entirely, and the mix-up is so common now that you will see both. And in joking, slightly theatrical use among friends: someone leaves the room, waves, and says 'adieu!' with a smile.

It is also a noun: 'to bid someone adieu' means to say goodbye to them, and 'she made her adieus' means she said her goodbyes. That noun use is old-fashioned and mostly literary.

How to say it: roughly 'a-DYOO' or 'a-DOO' in English. The plural, if you ever need it, is adieus or adieux, both accepted.

Though actually — the real weight of the word is in what it once meant. It is a short form of a French phrase meaning 'I commend you to God' — the sort of thing you said when someone rode away and you were not sure you would see them again. That is why it still sounds heavier than 'bye', even to people who do not know the history.