“ 1. mutually or reciprocally active, and
2: involving the actions or input of a user; especially: of, relating to, or being a two-way electronic communication system (as a telephone, cable television, or a computer) that involves a user's orders (as for information or merchandise) or responses (as to a poll)”
This is a fairly good definition but for the purpose of HCI is can be developed in many ways, and that is what I am trying to do. For instance, it is clear to me that the notion of agency needs to be included, maybe also the concept of predictability. Anyway, at the moment I am trying to find already existing definitions. If you have any suggestions of well developed definitions of interactivity, please let me know.