access control definition. Security feature that controls access to files and directories. Consists of a list of access permissions and user specifications. (For example, R,W,X:@.PAYROLL gives all users in the PAYROLL account read, write, and execute access to the file or directory that is assigned this ACD.) ACDs are applied to files or directories by using the ALTSEC command. By default, all files existing outside the traditional MPE account/group structure and all directories are assigned ACDs when they are created.