Refers to how much a system can be expanded. The term by itself implies a positive capability. For example, ?the device is known for its scalability? means that it can be made to serve a larger number of users without breaking down or requiring major changes on procedure. Will the application handle the expected user-load and beyond?
of a Web application is its capacity to deal with increasing traffic from users without showing any deterioration in performance. The scalability of a Web application is related to both its stability and its responsiveness.
See also Scalability Testing
or this interesting article∞