Stands for "Access Control List." An ACL is a list of user permissions for a file, folder, or other object. It defines what users and groups can access the object and what operations they can perform. These operations typically include read, write, and execute. For example, if an ACL specifies read-only access for a specific user of a file, that user will be able open the file, but cannot write to it or run the file.
Access control lists provide a straightforward method of managing file and folder permissions. They are used by most operating systems, including Windows, Mac, and Unix systems. While ACLs are typically hidden from the user, they can often be modified using a graphical interface. For example, in Mac OS X 10.5, ACL settings can be edited by selecting a file or folder and choosing "Get Info." The access control settings can be modified within the "Sharing and Permissions" section of the window. On Unix systems, ACLs can be edited using the chmod() command.