yeast starter means: Before yeasts or other leaveners became commercially accessible, yeast starters were used frequently. A mixture of water and flour with sugar and commercial yeast is used to create yeast starters. This allows for natural yeasts to be captured. Once the mixture is fermented, it is used as a part of a recipe. The remaining amount is then replenished by equal quantities of flour and water. You can replenish the starter and keep it going for an indefinite time. This method is popular for making sourdough bread. (in Cooking Dictionary)