Sourdough
Bread raised by a culture of wild yeasts and bacteria instead of packaged yeast.
Also: levain, wild yeast bread
Sourdough bread is leavened by a starter, a mixture of flour and water in which wild yeasts and lactic acid bacteria live. The yeasts make the gas that lifts the dough. The bacteria make acids that give the bread its mild sourness and help it keep longer. Before commercial yeast was sold in the nineteenth century, most leavened bread was made this way. The dough rises slowly, often overnight. Rye breads of northern and eastern Europe depend on sourdough.