I agree that people consider anything that appears in a web browser is a "web app", but that doesn't make it right. ActiveX, Java Applets, Silverlight, Flash, and NaCL are not "web apps." People also think "Internet" is analogous to "World Wide Web."
A "web app" is something that is built using open standards that has oversight by a standards committee that is either the W3C or WHATWG.
Feel free to peruse through WHATWG's specification for "Web Applications", or "Web Apps" for short. There is a standardized, very explicit definition of what that term means.
A "web app" is something that is built using open standards that has oversight by a standards committee that is either the W3C or WHATWG.