A scalable architecture refers to a system, network, or process that is designed to handle a workload that may change in scope. Which means the architecture can natively handle such growth, or that enlarging the architecture to handle growth is a trivial part of the original design.
Generally speaking, a computing system whose performance improves proportionally to hardware upgrades is a scalable system. An algorithm, protocol, or program is said to "scale" if it is suitably efficient and practical when applied to massive situations. Some of these instances include large data sets or numbers of users, high numbers of inputs or outputs, or a large number of nodes in a distributed system, for example.