<hardware, storage> (MO) A plastic or glass disk coated with a compound (often TbFeCo) with special optical, magnetic and thermal properties. The disk is read by bouncing a low-intensity laser off the disk. Originally the laser was infrared, but frequencies up to blue may be possible giving higher storage density. The polarisation of the reflected light depends on the polarity of the stored magnetic field.
To write, a higher intensity laser heats the coating up to its Curie point, allowing its magnetisation to be altered in a way that is retained when it has cooled.
The initial 5.25" MO drives, introduced at the end of the 1980s, were the size of a full-height 5.25" hard drive (like in IBM PC XT) and the disks looked like a CD-ROM enclosed in an old-style cartridge