Scalability
The ability of an IT service, process, configuration item, etc., to perform its agreed function when the workload or scope changes. Scalability can be vertical (scale up) or horizontal (scale out).
Impact
If you build a system that cannot scale, you are building a system that punishes you for success. When users come, the system dies.
Weinto take
Scale Out, not Up. We design stateless, shared-nothing architectures that allow us to add capacity simply by adding more nodes. Relying on "buying a bigger server" (vertical scaling) is a dead end; relying on distributed systems (horizontal scaling) is the sustainable strategy.