A web page is interactive when it prompts a response from the user.
Immediacy is the key to interactivity: If you click on a button that says "chat," then that experience would be considered "more interactive" if you are immediately able to chat with someone online. The experience would be "less interactive" if the response was an email address for a mailing list on which you are allowed to discuss a particular issue. Interactive is really just another code word for describing how graphical elements in web pages work together with the software behind them.
