You must be knowing that speed of an automobile is expressed in
terms of change in the position or distance covered by it in a certain
period of time Similarly, the speed of  a reaction or  the rate of a
reaction can be defined as the change in concentration of  a reactant
or product in unit time To be more specific, it can be expressed in
terms of:
(i) the rate of decrease in concentration of any one of the
reactants, or
(ii) the rate of increase in concentration of any one of the products Consider a hypothetical reaction, assuming that the volume of the
system remains constant