do you know what the central feature is that makes a system or at least part of a system a patient portal?

this would be the ability to expose PHI securely over the internet.  Patient portals have many uses and will vary by practice but they all allow patients to interact in some way with either their healthcare provider or staff.  It may be as simple as completing new patient forms prior to an appointment or a portal can be utilized by patients to review the results of their diagnostic studies or obtain copies of their personal record.