The term 'hair bun' can refer to two different hairstyles: a CHIGNON and a TOPKNOT. A CHIGNON is a classic hairstyle that involves gathering the hair into a low, neat bun at the nape of the neck. It is often seen in formal settings and is a popular choice for brides. A TOPKNOT is a more casual style that involves gathering the hair into a high bun on the top of the head. It is often seen in everyday settings and is a popular choice for those with long hair.