Rightsizing
The practice of matching compute, storage, and database instance sizes to actual workload demand, rather than to initial estimates or default configurations.
Definition
Rightsizing means adjusting the size or family of a resource, such as an EC2 instance, an RDS database, or an EBS volume, to match what the workload actually uses, based on observed CPU, memory, and I/O metrics rather than the estimate made when the resource was first provisioned. It's consistently one of the highest-impact, lowest-risk cost optimisation levers available, precisely because most environments are provisioned once for peak load and never revisited.
In Practice
An m5.2xlarge instance running at 8% average CPU utilisation for six months is a rightsizing candidate; moving it to an m5.large frees up cost without touching the application.