From Po Sa:
''The Presentation Abstraction Control architectural pattern (PAC) defines a structure for interactive software systems in the form of a hierarchy of cooperating agents.
Every agent is responsible for a specific aspect of the application's functionality and consists of three components: presentation, abstraction, and control.
This subdivision separates the human-computer interaction of the agent from its functional core and its communication with other agents.''
See www.idemployee.id.tue.nl 
page 27 for further explanation of the PAC pattern, as well as en.wikipedia.org
See also Hierarchical Model View Controller
See original on c2.com