The ability to automatically scale resources up or down in real time based on demand, and shrink them back when no longer needed to optimize cost.
One of the key cloud benefits in both AZ-900 and CLF-C02. Difference from scalability: elasticity implies automatic, bidirectional scaling. An e-commerce site on Black Friday is the classic example.
Elasticity is a concept that can show up on the exam. Check the full guide to see it in context.