<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.
Although optical, they appear as hard drives to the operating system and do not require a special filesystem (they can be formatted as FAT, HPFS, NTFS, etc.).
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
In 2006, a 3.5" drive has the size of 1.44 megabyte diskette drive with disks about the size of a regular 1.44MB floppy disc but twice the thickness.
Try this search on Wikipedia, OneLook, Google
Nearby terms: magnetic stripe « magnetic tape « magnetic tape drive « magneto-optical disk » magneto-optical drive » magnetostrictive delay line » MAGNUM