The capability of a medium to facilitate a two-way communication between people or between the user and the medium itself. More specifically, it is a chain process in which input and feedback are mutually consistent and meaningful and where the interlocutor/interlocutors is/are effectively engaged. In computer science, interactivity is understood as the dialog that occurs between a person and a computer programme/tool. Such interactivity is assured if the human users are motivated, engaged and enabled to express themselves by the tool.