Define Order of a reaction. Illustrate your answer with an example.

Order : The sum of the powers of the concentration terms of the reactants in the rate equation is called order of the reaction.
Ex: ${\mathrm{N}}_2{\mathrm{O}}_5\to {\mathrm{N}}_2{\mathrm{O}}_4+\frac{1}{2}{\mathrm{O}}_2$
$\mathrm{r}=\mathrm{k}{\left[{\mathrm{N}}_2{\mathrm{O}}_5\right]}^1$
Since sum of powers is one it is first order reaction.