A quintile is the portion of a frequency distribution containing a fifth (or 20%) of the total sample or population. The first quintile contains the lowest 20% of the sample or population. The first and second quintiles contain the lowest 40% of the sample or population.
A quintile is an example of a quantile. A quantile is: “Each of any set of values of a variate which divide a frequency distribution into equal groups, each containing the same fraction of the total population” (Oxford Dictionary).