What is scalability in network architecture?

The ability to grow quickly to support new users and applications without degrading service for existing users.

A scalable network can expand quickly and easily to support new users and applications without impacting the performance of services to existing users.

How achieved: Network designers follow accepted standards and protocols to make networks scalable.

Go deeper:

• The Network as a Platform (Cisco Press) — ITN's own definition: expand quickly without degrading performance, enabled by standards.
• Scalability (Wikipedia) — deeper general treatment: handling growing workloads, scale-out vs scale-up, dimensions of scalability.