disk

A disk is a flat, circular object with uniform thickness. In geometry, a disk refers to the set of all points in a plane whose distance from a fixed center is less than or equal to a given radius. The area of a disk is πr², and its circumference is 2πr. If extended into a cylinder of height h, its volume is πr²h.

In computing, the term disk commonly denotes a data storage device that records digital information on magnetically charged surfaces or on solid-state memory. Hard disk drives (HDDs) use spinning platters with read/write heads and rely on magnetic storage; performance is influenced by rotational speed, seek time, and data density. Solid-state drives (SSDs) use non-volatile flash memory, offering faster access and no moving parts, but traditionally at higher cost per gigabyte.

Optical disks are another storage medium, including CDs, DVDs, and Blu-ray discs. Data are encoded as microscopic pits and lands on a reflective layer beneath a protective coating. Reading is achieved with a laser. Typical capacities are about 700 MB for a CD, 4.7 GB for a single-layer DVD, and 25–50 GB for a single- to dual-layer Blu-ray disc.

Removable disk media such as floppy disks were once common but have largely been replaced by USB flash drives, external SSDs, and cloud storage. In computing, disk storage is often presented as volumes or partitions formatted with various file systems.

Beyond technology, disk is also a geometric term and appears in astronomy as the flat, rotating disk-like component of a spiral galaxy, known as the galactic disk.