The ability for something designed to operate at one measure of size to operate successfully at other sizes. The term is commonly used in relation to the development of shared computer applications that are intended to be used by large numbers of users. Of necessity, developments take place with a small number of test users. Unless the application is carefully designed to take account of the interactions that will arise when it is called on to service a large number of users, it may well fail to operate at all, or to operate only with an unacceptable level of service. An application that successfully expands its numbers of supported users is said to be scalable.