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

Mathematical Sciences

Abstraction
Anthropics
Category Theory
Causality
Game Theory
Decision Theory
Information Theory
Logic & Mathematics
Probability & Statistics

Specifics
Prisoner's Dilemma

General Science & Eng

Machine Learning
Nanotechnology
Physics
Programming
Space Exploration & Colonization

Specifics
The Great Filter

Meta / Misc

Academic Papers
Book Reviews
Distillation & Pedagogy
Fact Posts
Research Agendas
Scholarship & Learning

Social & Economic

Economics
Financial Investing
History
Politics
Progress Studies
Social and Cultural Dynamics

Specifics
Conflict vs Mistake Theory
Cost Disease
Efficient Market Hypothesis
Industrial Revolution
Moral Mazes
Signaling
Social Reality
Social Status

Biological & Psychological

Aging
Biology
Consciousness
Evolution
Evolutionary Psychology
Medicine
Neuroscience
Qualia

Specifics
Coronavirus
General Intelligence
IQ / g-factor
Neocortex

The Practice of Modeling

Epistemic Review
Expertise
Gears-Level Models
Falsifiability
Forecasting & Prediction
Forecasts (Lists of)
Inside/Outside View
Jargon (meta)
Practice and Philosophy of Science
Prediction Markets
Reductionism
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, ...