In chemistry, the limiting reagent is the reactant that is completely consumed in a chemical reaction. When the limiting reagent is gone, no more product can be formed -- and that's how this component earned its name.
The limiting reagent is needed to calculate the percentage yield of a chemical reaction's product. This is done through the application of balanced chemical equations (this branch of chemistry is known as stoichiometry). Given a balanced chemical equation, a chemical reaction will always have a constant ratio of reagents to product. Whichever reactant is not completely consumed by the chemical reaction is known as the excess reagent.