World : The complete description of the simulation ( can contain static or dynamic objects, scenes, plugins, GUI etc). It is contained in a SDF (Simulation Description Format) file.

Entity : An 'object' in the world, defined by a Numeric ID provided at run time, it contains components inside it.

Component : It is something that gives a charateristic or fucntionality to the Component.(Pose, name, material etc)

System : It is basically a logic that is applied/operates on all entities having some predifined components. These are runtime loadable plugins.

ECM - Entity Component Manager : A class providing API for dealing with functionalities like creating, removing, updatinig and querying entities.

Level : Part of the world defined by a box volume and static entities inside it. Levels may overlap each other and an entity may be present in one or more levels.

Buffer Zone : Inflation of the level's volume outside its boundaries. used to detect whether a performer (entity) belong to a particular level or not.

Performer : A moving entity that change levels during simulation (robots, actors or dynamic models). A performer is meaningful only if there are levels.

Global Entity : An entity that is present on all level. eg: light source -> sun, ground plane, height maps etc.

Default Level : Level which handles all entities that are not within any other levels.

Network Manager : A manager of information flow across distributed simulation processes. For a distributed simulation there are Pirmary manager and multiple Secondary Managers. The primary ensures that all the secondary managers are in sync. If the simulation is not distributed then there is no need for network managers.

Event Manager : Manages events that can be sent across systems and servers.

Simulation Runner : Runner that runs simulation ( world or some levels) no support for running multiple worlds parallely, contains a single ECM, event maanger, network manager(if simulation is distributed), loads up a set of systems.

Server : The entry point for ignition gazebo. It loads the SDF file and instantiates simulation runner per world.