undo (n)~ The phase during database recovery that reverses (rolls back) changes made by any transactions that were uncommitted when the redo phase of recovery completed. undo disk (n)~ A virtual hard disk that stores the changes made to a virtual machine’s guest operating system and data after the Undo Disks feature has been enabled for the virtual machine. Changes accumulate in the undo disk (.vud) file until you commit or discard the changes. Each time you shut down or turn off a virtual machine you decide whether to keep commit or discard the changes.