Terminology

Engine

is defined as MM::Engine.

Service

is defined as MM::Services::Service, the Service's base class, which was previously known as ServiceSystem and SubSystem.

Scene

is defined as a ECS-style EnTT::registry. It contains Entities, Components and Systems.

Entity

is defined as a ECS Entity, see EnTT.

System

is defined as a ECS-style System. In this case, it's a free function.

Screen

is defined as a State of enabled and disabled Services. (see ScreenDirector)