Define 'scalability' in the context of data center design.

Scalability in the context of data center design refers to the capability to expand resources without disruption. This means that as demand for computing power, storage, or network bandwidth increases, a data center can add the necessary resources—be it hardware or software—without causing significant downtime or requiring extensive reconfiguration of existing systems.

This characteristic is crucial for data centers, as businesses often experience fluctuating workloads and need the flexibility to adapt quickly to changes in demand. For example, if a company experiences a sudden surge in traffic, a scalable data center can accommodate this increase smoothly by provisioning additional servers or storage without interrupting service or affecting existing operations.

In contrast, the other options focus on aspects that do not fully encapsulate the idea of scalability. Adding more physical space may be a part of the process but does not necessarily address the broader aspect of operational efficiency during expansion. Assessing system performance or measuring energy efficiency pertains to separate areas of data center management that do not directly relate to how easily a data center can grow with its demands.