AWS's container orchestration service. It manages the lifecycle of Docker containers on EC2 or Fargate clusters. An alternative to Kubernetes with lower operational complexity.
This shows up as a container service on CLF-C02 and SAA-C03. Difference from EKS: ECS is AWS's own orchestrator; EKS is managed Kubernetes. With Fargate you don't have to manage the underlying nodes.
Amazon ECS (Elastic Container Service) is a concept that can show up on the exam. Check the full guide to see it in context.