A bureikō is a banquet held without regard to rank or etiquette. The most famous one in Japanese history was held by Emperor Go-Daigo in the Kamakura period. He used it to gather people of different ranks and plot in secret. One guest told his wife, and the plot leaked.
The same source notes that, in practice, rank does not disappear at a bureikō. Communication at company drinking parties has its own coined word, nominication, from nomu (to drink). It first appeared in the media in the 1980s.