Kubernetes Operators

Operators are applications that enable us to extend the Kubernetes API with its domain-specific knowledge.
For example, an operator that manages the creation and deletion of databases, by monitoring the desired state in custom resources (within a Kubernetes cluster), and matching it to the actual state.

Here is another description of operators from the Kubernetes documentation:
“Operators are software extensions to Kubernetes that make use of custom resources to manage applications and their components. Operators follow Kubernetes principles, notably the control loop.”