ACL or Access Control List is a list of permissions attached to an object. It specifies which users/system processes have access to the object as well as what they are allowed to do.
ACLs (Access Control Lists)
(more information on this Wikipedia page)
An ACL is a table of users or groups that are allowed to access system objects. The entries in the table are called ACEs or access control entitles in Microsoft Windows NT1, OpenVMS, Unix-like, and macOS operating systems. The permissions define if a user can read, write or execute the object.
An ACL can also refer to a networking ACL. Networking ACLs are rules applied to ports or network daemon names with a list of hosts/names that are allowed to access the port.