People toss around the terms "bun" and "chignon" a lot, but are the two interchangeable? No, actually, at least not in English. "Chignon" is the French word for bun, but in its Australian and British usage, it's an abbreviated term, originating from the phrase "chignon du cou," or "bun at the nape of the neck." But you can become an expert on the differences between the two now.
Here's the long and short of it: in current jargon, while many chignons are buns, few buns are chignons. Buns can be anywhere on your head and you can wear lots of them at once. You can have them looped around your ears Princess Leia-style, stuck up on your crown and worn messy, small, whatever.
Chignons, on the other hand, have traditionally been worn low on the head. They're also considered more formal, and you only wear one, usually large and containing the entire length of your hair.
The other notable difference? Buns are always wrapped around themselves, whether twisted around a centre or braided. Chignons, however, aren't necessarily so. They may simply be tucked under or can be built around a central pin, tiara, or other hair accessory.
So while "chignon" technically just means a low bun, the word also carries connotations of formality, as well as being used to describe up 'dos that aren't buns. So there you have it.