A cylinder of very dense, very pure glass that is formed as a preliminary step in the manufacture of optical fiber. The cylinder is formed of vaporized glass and dopants that have cooled and settled on a bait rod in a process known as outside vapor deposition (OVD).The bait rod is then separated, and the cylinder is heated and collapsed to form the preform cylinder, which then is heated in a drawing tower. A gob of molten glass then forms and is drawn by gravity into an optical fiber. See also OVD.