AWS infrastructure-as-code framework for defining resources in programming languages such as TypeScript, Python, Java, C# or Go. The code is synthesized into AWS CloudFormation templates, which are what actually deploy the resources.
In SAA-C03 and SAP-C02 it is compared with CloudFormation: both are IaC and CloudFormation performs the deployment, but with CDK you write code with loops, conditionals and reusable components (constructs) instead of YAML or JSON. Question: the team wants to define infrastructure in TypeScript and deploy it with CloudFormation → AWS CDK.
AWS CDK (Cloud Development Kit) is a concept that can show up on the exam. Check the full guide to see it in context.