The Joomla! Glossary is helpful for learning common terms used in Joomla! tutorials, help screens and advanced documentation.
Access Control List or ACL is according to the Wikipedia definition, “...ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed to be performed on given objects.” In the case of Joomla there are two separate aspects to its Access Control List which site administrators can control:
The implementation of ACL in Joomla was substantially changed in the Joomla! 2.5 series which allowed for more flexibility in groups and permissions.