Correlation
How closely two assets move together, on a scale from +1 (in lockstep) through 0 (unrelated) to −1 (opposite). It's the engine of diversification — combining assets that aren't perfectly correlated is what lowers a portfolio's risk.
Where this is taught
Definitions are the trailer. These free levels turn Correlation into something you play — one bite-size lesson, with worked examples, a quiz and XP.