Odango (お団子) is a reference to a Japanese dumpling, and is used to describe hairstyles in which the woman's hair is bound in spherical or nearly-spherical buns on her head. "Odango atama" (お団子頭) means "dumpling head," and is a way of insulting people with that hairstyle. Since dumplings are not as common in North America, in the English dub the term was changed to "meatball head" or "bun head," while the Kodansha release of the English-translated manga called it either "bump head" or "bun head."
In the Sailor Moon manga and anime, this hairstyle was worn by all members of the Moon matriarchy, Diana and Luna in their human forms, Chibi Chibi, the Maenads, several villians including Cyprine, Ptilol, Tellu, and Queen Nehellenia, and the unused character, Diana the Moon Fairy.
In the live-action series, Usagi wore this hairstyle in her civilian identity and as Sailor Moon, but not in her Princess Serenity form. Sailor Venus had an odango hairstyle similar to Queen Nehellenia's, though her odango were much smaller and not as obvious. Luna in her human form had an odango hairstyle similar to Sailor Moon's, but in her Sailor Luna form the odango were replaced by her cat ears, leaving her with just pigtails.