What is "kotatsu"

n. A knee-high table with an electric foot-warmer installed inside on the top board, which is used with a coverlet during winter.

A is a low, wooden table frame covered by a futon, or heavy blanket, upon which a table top sits. Underneath is a heat source, formerly a charcoal brazier but now electric, often built into the table itself. Kotatsu are used almost exclusively in Japan, although similar devices are used elsewhere.