Discs

Discs, or discs, are flat, circular objects whose diameter is typically much larger than their thickness. The term covers a range of physical objects as well as mathematical surfaces, and it varies in meaning by context.

In geometry, a disc is the filled interior of a circle: all points at distance less than or equal to a given radius from a center. The boundary, a circle, consists of points at exactly that radius. This distinction between the disc and its boundary, the circle, is fundamental in planar geometry.

In everyday use, discs appear as physical objects and components. Examples include metal brake discs and clutch discs in machinery and vehicles, flywheels, as well as vinyl records and optical storage discs such as CDs, DVDs, and Blu-ray discs. In computing and data storage, the term disc is common for optical formats, though disk is also widely used, especially for magnetic storage like hard disks and floppy disks.

In biology, intervertebral discs cushion the vertebrae and facilitate movement; damage or degeneration can contribute to back pain and reduced mobility. In astronomy and astrophysics, accretion or protoplanetary discs are rotating structures of gas and dust around young stars or black holes, playing key roles in star and planet formation.

Spelling varies by region: disc is preferred in British English, while disk is common in American English. Both forms denote the same broad concept of a flat, circular object.