A statistical value of a data set that represents 20% of a given population. The first quartile represents the lowest fifth of the data (1-20%); the second quartile represents the second fifth (21% - 40%) etc.
Quintiles are often used to create cut-off points for a given population. For example, a government sponsored socio-economic study may use quintiles to determine the maximum wealth a family could possess in order to belong to the lowest quintile of society.
This cut-off point can then be used as a prerequisite for a family to receive a special government subsidy aimed to help society's less fortunate.