To me interactivity is a two way transfer of information between two entities. For example if person reads a book there is a one way transfer of information but if he flips the page the state of the book changes. In a sense even reading a book can be considered interactive.
What I am personally interested in are video games where the actions of the players influence the state of the game world. Games like the Witcher series where the actions of the player affect the game even the actions from previous games affect the later ones.