ACT-R is a model of the human cognitive process developed and used by cognitive psychologists, which can be applied to HCI. It is an acronym for "The Adaptive Control of Thought - Rational". While it is often referred to as "the ACT-R theory", it is not properly considered a theory of cognition, but rather a cognitive architecture that can accommodate different theories. The scope of ACT-R is greater than the scope of any particular theory, and multiple (possibly competing) theories can fit within the framework of ACT-R. It was developed to model problem solving, learning and memory.
P. Pirolli, and W. Fu. User Modeling 2003 9th International Conference, UM 2003 Johnstown, PA, USA, June 22–26, 2003 Proceedings, volume 2702 of Lecture Notes in Computer Science, page 45-54. Heidelberg, Springer, (2003)