.. SPDX-FileCopyrightText: 2026 James Turner <james@flightgear.org>
.. SPDX-License-Identifier: GPL-2.0-or-later

Common Terms
============

.. glossary::

    Subsystem
        A subsystem is as mostly independent processing component, with
        well defined lifetime semantics. Each main loop iteration, all
        subsystems are updated, before the next frame is rendered.

        Subsystems can be simple and self-contained, or they can create
        entire hierarchies of objects and elements. They can do
        networking, IO, arbitrary computation, or anything else. Because
        they run synchronously with the main loop, they should not take
        large amounts of CPU time during their
        :cpp:func:`SGSubsystem::update()`.

    Property
        A property is a named, typed value which can be read and written
        across the simulator. Because properties can be easily used in
        C++, scripts and XML expressions, they are the main way to move
        information around inside the simulator.

    PropertyList
        A property list is a valid XML document whose root (document element)
        is `PropertyList`. The file defines a tree of property nodes.

    Command
        A command is a named piece of functionality, implemented in C++
        or Nasal. Commands have a unique name and take a collection of
        properties as arguments.

    Binding
        Bindings are a standard XML concept to trigger a :term:`command`
        when a certain event occurs. Typically the binding defines various
        arguments, and in some cases runtime values are also included
        (e.g., when a joystick axis binding is triggered)