World Modeling is getting curious about how the world works. It’s diving into wikipedia, it’s running a survey to get data from your friends, it’s dropping balls from different heights and measuring how long they take to fall. Empiricism, scholarship, googling, introspection, data-gathering, science. Applying your epistemology and curiosity, finding out how the damn thing works, and writing it down for the rest of us.

The eleventh virtue is scholarship. Study many sciences and absorb their power as your own. Each field that you consume makes you larger. If you swallow enough sciences the gaps between them will diminish and your knowledge will become a unified whole. If you are gluttonous you will become vaster than mountains.
-- Twelve Virtues of Rationality

World Modeling Sub-Topics

Biology
Causality
Consciousness
Economics
Cost Disease
History

Machine Learning
Probability & Statistics
Programming
Social Reality

Fact Posts
Progress Studies
Epistemic Review
Replicability

A definition by elimination

Properly considered, the overwhelming majority of content LessWrong is about modeling how the world is, including almost all posts on Rationality and all practical advice. The intended usage of World Modeling is to capture all content describing how the world is that is not captured by the more specific major tags of Rationality, World Optimization, and AI.

The Rationality tag is for content that is about how the world is in relation to how minds works and what one ought to do in order to reach true beliefs. The question for that category is does this relate to how I ought to think?
The World Optimization tag is for content about how the world is which is relevant to choosing actions in a relatively immediate way. By this definition, it encompasses most posts discussing altruistic methods and targets, as well practical personal advice. The question for that category is is this content motivated by the desire to optimize the world?
The AI tag is for content about how the world is which is relevant to questions of how advanced artificial intelligence will affect the world and how to ensure outcomes are good. The question is does this help me make predictions about AI or ensure AI will have good outcomes?

If content warrants a no to all of the above question...

Human Genetics