Intentionality, originally a concept from scholastic philosophy, was reintroduced in contemporary philosophy by the philosopher and psychologist Franz Brentano in his work Psychologie vom Empirischen Standpunkte. While often simplistically summarised as "aboutness" or the relationship between mental acts and the external world, Brentano defined it as the main characteristic of "psychical phenomena" (psychische Phänomene), by which they could be distinguished from "physical phenomena" (physische Phänomene). Every psychical or mental phenomenon, every psychological act has a content, is directed at an object (the intentional object). Every belief, desire etc. has an object that it is about: the believed, the wanted. Brentano used the expression "intentional inexistence" to indicate the status of the objects of thought in the mind. The property of being intentional, of having an intentional object, was the key feature to distinguish psychical phenomena and physical phenomena, because physical phenomena lack intentionality altogether.

Through the works of Husserl, who took it over from Brentano, the concept of intentionality received more widespread attention in current philosophy, both continental and analytic.

In current artificial intelligence and philosophy of mind it is a controversial subject and thought to be something that a machine will perhaps never achieve.

In the field of social cognition and the study of folk psychology, intentionality has a different meaning. Human perceivers consider a behavior intentional when it is appears purposeful or done intentionally -- that is, based on reasons (beliefs, desires) and performed with skill and awareness. In many contexts, people read the intentions underlying others' behavior effortlessly. For more information:

Malle, B. F., Moses, L. J., & Baldwin, D. A. (Eds.). (2001). Intentions and intentionality: Foundations of social cognition. Cambridge, MA: MIT Press.
N.B. intentionality (-tion-) is not to be confused with intensionality (-sion-), a concept from semantics.