Idiosyncratic risk (also called specific risk or unsystematic risk) is the portion of an asset's volatility that is unique to that company or asset, separate from broader market forces. Examples include an unexpected earnings miss, a CEO scandal, a product recall, a patent lawsuit, or a surprise acquisition.
This type of risk is fundamentally different from factor (systematic) risk in one critical way: it is diversifiable. By holding a sufficiently large, uncorrelated portfolio of assets, idiosyncratic risks cancel each other out. This is the theoretical foundation of Modern Portfolio Theory.
On the StressTest.pro Risk X-Ray, the 'Idiosyncratic' slice of the donut chart shows what fraction of the asset's total estimated volatility comes from company-specific factors. A single-stock position with 60% idiosyncratic risk is highly concentrated and sensitive to company events. A broad index ETF like VOO will show near-zero idiosyncratic risk since diversification has effectively eliminated it.