The system can handle growth in users, data, or traffic by adding resources — not by needing a redesign.
Mental model: Like a restaurant chain built to handle a rush of 500 customers vs. a food truck serving 20 an hour.
Example: An app that handles a traffic spike by spinning up ten more server instances instead of needing a rewrite.
In practice: Horizontal scaling (more machines) and vertical scaling (a bigger machine) solve this differently — horizontal scaling usually requires the system to be stateless or partition-aware first.