A pre-ferment made with equal parts flour and water plus a tiny amount of commercial yeast, fermented for 8-16 hours before being added to the final dough.
Poolish is a technique from French baking tradition that bridges the gap between commercial yeast and sourdough. It gives yeasted bread some of the flavor benefits of long fermentation without requiring a sourdough starter. A poolish typically uses about 0.1-0.5% yeast (relative to the poolish's flour), giving it time to develop flavor before being mixed into the final dough. It's not sourdough, but it's a step in the right direction.
