Entity: An entity represents a general-purpose object. In a game engine context, for example, every coarse game object is represented as an entity. Usually, it only consists of a unique id. Implementations typically use a plain integer for this.[1]
Component: A component labels an entity as possessing a particular aspect, and holds the data needed to model that aspect. For example, every game object that can take damage might have a Health component associated with its entity. Implementations typically use structs, classes, or associative arrays.[1]
System: A system is a process which acts on all entities with the desired components. For example a physics system may query for entities having mass, velocity and position components, and iterate over the results doing physics calculations on the sets of components for each entity.
The behavior of an entity can be changed at runtime by systems that add, remove or modify components. This eliminates the ambiguity problems of deep and wide inheritance hierarchies often found in Object Oriented Programming techniques that are difficult to understand, maintain, and extend. Common ECS approaches are highly compatible with, and are often combined with, data-oriented design techniques. Data for all instances of a component are commonly stored together in physical memory, enabling efficient memory access for systems which operate over many entities.
