there are two distinct definitions of ECS, the first one given in https://news.ycombinator.com/item?id=27856030 https://news.ycombinator.com/item?id=27856030 is

there are two distinct definitions of ECS, the first one given in https://news.ycombinator.com/item?id=27856030 https://news.ycombinator.com/item?id=27856030 is the "(entity-component-system) architecture" where there are entities, components, and systems, as distinct objects in your software. The second is "(entity-component) system" or "entity-component architecture" where the behaviour goes inside components (for instance, Unity3D gameobjects with their components).