Infrastructure as code
Also known as: IaC
Infrastructure as code means describing servers, networks, databases and their configuration in files that are version-controlled and applied by a tool, rather than assembling them by clicking through a provider's console.
The point is reproducibility. The description is reviewable, its history explains why anything is the way it is, and the same files can build an identical staging environment or rebuild production after a disaster. Infrastructure that exists only as the result of past clicking is infrastructure nobody can safely change.
It also removes the specific and common failure where the person who set everything up leaves. What they knew is in the repository rather than in their memory — which is the same reason the deployment of a site should be a script and not a runbook of remembered steps.