What it is:
A quintile is one of five equal parts.
How it works (Example):
For example, if we were to look at all of the closing prices for Company XYZ stock for every day in the last year, the top 20% of those prices would represent the upper quintile of the data. The bottom 20% of those prices would represent the lower quintile of the data. There would be three quintiles in between the upper and lower quintiles. The average of all the stock prices typically falls right in the middle, between the second and fourth quintiles.