Vellum is prepared animal skin or membrane, typically used as writing material. Parchment is another term for this material, from which vellum is sometimes distinguished, when it is made from calfskin, as opposed to that made from other animals, or otherwise being of higher quality. Vellum is prepared for writing or printing on, to produce single pages, scrolls, codices, or books.

Modern scholars and custodians increasingly use only the less specific, potentially-confusing term "membrane". Depending on factors such as the method of preparation it may be very hard to determine the animal species involved (let alone its age) without using a laboratory, and the term avoids the need to distinguish between vellum and parchment.

Vellum is generally smooth and durable, although there are great variations depending on preparation and the quality of the skin. The manufacture involves the cleaning, bleaching, stretching on a frame (a "herse"), and scraping of the skin with a crescent-shaped knife (a "lunarium" or "lunellum"). To create tension, scraping is alternated with wetting and drying. A final finish may be achieved by abrading the surface with pumice, and treating with a preparation of lime or chalk to make it accept writing or printing ink.

Modern "paper vellum" is made of synthetic plant material, and is called such for its usage and quality similarities. Paper vellum is used for a variety of purposes including tracing, technical drawings, plans and blueprints.