Ergodicity — pronounced ur-guh-DISS-uh-tee. In probability and dynamical systems, it is the property that lets observations along one sufficiently long trajectory represent the distribution across many possible realizations, under the model’s conditions. A useful sentence: “The backtest quietly assumed ergodicity, treating one market history as though it represented the whole distribution.” The useful caution is that a long sample does not make the assumption true. Cambridge pronunciation and definition