The coordinate system for world space or the model space as a whole.
A book in object space rests on a table in world space. The table uses the world coordinate system.
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World space is the universal coordinate system for all objects in the scene. When you look at the home grid in the viewports, you see the World Space coordinate system. World space is constant and immovable.
In the world coordinate system seen from the front, the X axis runs in a positive direction to the right, the Z axis runs in a positive direction upward, and the Y axis runs in a positive direction away from you.