DVD-9 - Computer Definition
A DVD with two data layers on one side. DVD-9 media have nearly twice as much storage as a regular DVD disc (8.5 vs. 4.7GB). DVD-R DL (minus R) typically uses the "dual layer" designation, while DVD+R DL (plus R) uses "double layer." However, the "dual layer" and "double layer" designations are often used synonymously. See DVD-5 and DVD storage capacities.