An ancient material that,as far as we are aware, pre-dates paper, whereby animal skin (sheep or goat) is treated until it is thin and fine enough to write on. Parchment is also used for upholstery, to cover boxes and photo frames, for the details on small items and for lampshades. The light that comes through parchment is second to none in terms of diffusion, and in warmth, softness and effect similar only to candlelight.
In loose terms it is also sometimes used to describe anything that recalls the look and/or feel of original parchment.
As a material, parchment is extremely durable, long lasting and because it absorbs ink right to its core, surviving fragments of written parchments, or scrolls, can still be reliably read, millennia after they were written. Many will know of the Dead Sea scrolls, the collection of mostly parchment texts from the Hebrew Bible written over 2000 years ago, which were discovered and re-covered in the 20thC.
See also Vellum.