Pure ECS can also describe one of thw two ways that Unity allows developers to create projects using their--as of 2020, still in development/in preview--ECS development workflow: Hybrid or Pure.
In a project that is written with Pure ECS, each game object is built purely with Entities, Components, and Systems. GameObjects and Components/Monobehaviors are predominantly not used. (This is not always true as there are still certain types of objects that are unavailable in an ECS form and may still need to be created as traditional Unity GameObjects. I imagine by the first release of ECS there will be ECS form of everything needed to build most games.)
Alternatively, Hybrid ECS is being offered as a go-between the classic GameObject/Monobehavior pattern and the ECS pattern. In this, the developer can use GameObjects and attach a special form of Monobehavior that converts GameObjects into Entities and certain Monobehaviors into Components.