In stoichiometry of a chemical reaction to produce a chemical product, it may be observed or predicted that with amounts supplied in specified proportions, one of the reactants will be consumed by the reaction before the others. The amount of product is thus limited by the supply of this reagent. This limiting reagent determines the theoretical yield of the reaction.  The other reactants are said to be non-limiting or in excess.  This distinction makes sense only when the chemical equilibrium so favors the products to cause the complete consumption of one of the reactants.
In studies of reaction kinetics, the rate of progress of the reaction may be limited by the concentration of one of the reactants or catalyst.  In multi-step reactions, a step may be rate-limiting in terms of production of the final product.  In vivo, in an organism or in an ecologic system, such factors as those may be rate-limiting, or in the overall analysis of a multi-step process including biologic, geologic, hydrologic, or atmospheric transport and chemical reactions, transport of a reactant may be limiting.