In chemistry, yield, also referred to as reaction yield, is a measure of the quantity of moles of a product formed in relation to the reactant consumed, obtained in a chemical reaction, usually expressed as a percentage.  Yield is one of the primary factors that scientists must consider in organic and inorganic chemical synthesis processes. In chemical reaction engineering, "yield",  "conversion" and "selectivity" are terms used to describe ratios of how much of a reactant was consumed (conversion),  how much desired product was formed (yield) in relation to the undesired product (selectivity), represented as X, Y, and S.
The term yield also plays an important role in analytical chemistry, as individual compounds are recovered in purification processes in a range from quantitative yield (100 %) to low yield (< 50 %).