n. straw matting, in a standard size, used as a floor covering in Japanese houses
The Century Dictionary and Cyclopedia
n. A floor-mat about two inches thick, made of rice-straw bound together and covered on the upper surface with matting. The edges are usually bound with cloth.
n. A Japanese measure of surface, that of a mat 6 shaku in length by 3 shaku in width, or nearly 6 feet by 3 feet.
Word Usage
"Japan, of course, uses that oh-so-traditional unit of measurement known as the tatami mat, and the size of a room is always expressed in how many tatami would fit inside, even if it's a traditional Western room with wooden flooring."