The reaction order is the relationship between the concentrations of species and the rate of a reaction.
The order of a rate law is the sum of the exponents of its concentration terms. Once the rate law of a reaction has been determined, that same law can be used to understand more fully the composition of the reaction mixture. More specifically, the reaction order is the exponent to which the concentration of that species is raised, and it indicates to what extent the concentration of a species affects the rate of a reaction, as well as which species has the greatest effect. For the N2O5 decomposition with a rate law of k[N2O5], this exponent is 1 (and thus is not explicitly shown); this reaction is therefore a first order reaction. It can also be said that the reaction is "first order in N2O5". For more complicated rate laws, the overall reaction order and the orders with respect to each component are used.