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1. The two-dimensional projection of the surface of a star or planet.
2. See Galaxy; galaxies.


A flat, circular, raised ornament, carved as a series of disks adjacent to each other.


Also spelled disc.
A relatively thin layer of material distributed in the central plane of a spiral galaxy, in contrast to the nucleus or halo.
Any of various rounded and flattened animal and plant structures.
(computer science)
A rotating circular plate having a magnetizable surface on which information may be stored as a pattern of polarized spots on concentric recording tracks. Also known as magnetic disk.
(engineering acoustics)
The region in the plane consisting of all points with norm less than 1 (sometimes less than or equal to 1).


1. magnetic disk.

2. compact disc.

3. optical disk.

Note: the american spelling, "disk", is normal for most computer disks whereas "compact disc", having come to computers via the audio world, is correctly spelled with a "c", indeed, this spelling is part of the CD standard.


A storage device that uses rotating platters divided into sectors that hold a fixed amount of data. The sectors are stored in tracks that are recorded in concentric circles on the platter. When a sector is read or written, a mechanical access arm moves to the required track (see access arm).

Invented in the 1950s and the primary storage medium since the 1970s, the disk has slowly but surely given way to solid state drives (SSDs) that have no mechanical parts. See sector, hard disk, magnetic disk, SSD, optical disc, CD-ROM, DVD, floppy disk, RAMAC and DASD.

Disk and Memory Work Together
On the disk, data are stored in sectors, which hold a chunk of data (typically 4,096 bytes) and are the smallest unit that can be read or written. Memory (RAM) is like a checkerboard, each square holding one byte. In RAM, the contents of any single byte or group of bytes can be calculated, compared and copied independently. See storage vs. memory and byte addressable.