Define the following terms:
$(1)$ Rate law / Rate equation / Rate expression
$(2)$ Unimolecular reaction

(N/A) $(1)$ Rate law (or rate equation or rate expression) is the expression in which reaction rate is given in terms of molar concentration of reactants with each term raised to some power,which may or may not be equal to the stoichiometric coefficient of the reacting species in a balanced chemical equation.
$(2)$ $A$ reaction is said to be unimolecular if only one reacting species is involved in the elementary step of the reaction,for example,the decomposition of $N_2O_5$ $(N_2O_5 \rightarrow N_2O_4 + \frac{1}{2}O_2)$.