The exact amount of reactant required to react with another element can be estimated using reaction stoichiometry.
If the reactants are not blended in the proper stoichiometric proportions (as specified by the balanced chemical equation), one reactant will be consumed completely while the other will be left over.
The limiting reagent is the one that has been completely consumed; it prevents the reaction from continuing because there is no longer any reactant to react with.
Final answer: Limiting reagent is the reactant that is totally consumed in a reaction, determining when the reaction comes to a halt.