The reactant that is completely consumed in a chemical reaction is called the limiting reactant (or limiting reagent) because it determines (or limits) the amount of product formed.
First write the balanced chemical reaction. One must determine the stoichiometry of the reaction. The limiting reagent is the chemical which will run out first as the reaction occurs and the reactants are consumed. The other reactants, the ones are are leftover when the limiting reagent runs out, are said to be in excess.