Infrastructure as Code (IaC) is a well established paradigm and refers to the standard practice of treating infrastructure (network, disk, storage, databases, message queues etc.) in the same way as application code and applying general software engineering practices including source control versioning, testing and more. For example, Terraform and AWS CloudFormation are widely-adopted technologies that use configuration files/templates to represent the infrastructure components.
Infrastructure-IS-Code - A Different Way of Thinking About This
Imagine you have an application that comprises a Serverless function fronted by an API Gateway along with a NoSQL database as the backend. Instead of defining it in a static way (using JSON, YAML, etc.), one can represent these components using standard programming language constructs such as classes, methods, etc. Here is a pseudo-code example: