Rate of reaction:
The rate of a chemical reaction is defined as the velocity with which a reactant or reactants undergo chemical change.
Zero Order Reaction:
If the rate of a reaction is independent of the concentration of the reacting species, the reaction is said to be a zero-order reaction.
First order reaction:
A first order reaction is one where the rate depends on the concentration of the species to the first power.
Second order reaction:
A second order reaction is one where the rate depends on the concentration of the species to the second power. 
