A poolish (French pouliche
) is a yeast pre-ferment
or yeast starter that is used to leaven breads and also to give it added flavor.
As are most starters, a poolish is simply a mixture of flour, water, and yeast although it can sometimes also contain diastatic malt and sugar. It usually uses commercial yeast, if natural yeast is used then it is closer to a sourdough.
Compared to a biga or a levain levure, a poolish is more liquid. It gives bread that slightly nutty taste and aroma typical of well made french bread. Baguettes and similarly shaped breads (usually called french bread) are traditionally made with a poolish yeast starter.