A proprietry recordable media used for music, invented by Sony.

They're 7cm x 6.75cm x 0.5cm in size and the disc inside is 64 mm in diameter. They hold 160MB of data and use Sony's compression format ATRAC to store upto 74 minutes of audio.

Hi-MD discs (just released) can hold upto 1GB of data and support music in WMA, MP3 and (various flavours of) ATRAC codecs.