Price discrimination is the practice of charging different
prices to different consumers for the same good,
or charging different prices for different amounts bought. First-degree
or perfect price discrimination is the practice of charging
each consumer an amount equal to his willingness-to-pay.
Second-degree price discrimination or nonlinear pricing
is the practice of charging a consumer a price per unit that depends
on the purchase amount. Third-degree price discrimination
is the practice of charging different prices for the same good
to consumers belonging to different groups, depending on the price
elasticity of demand in each group.