Percentile is a convenient term for denoting thresholds or boundary values in frequency distributions. Thus the 5th percentile is that value which marks off the lowest 5 per cent of the observations from the rest, the 50th percentile is the same as the median, and the 95th percentile exceeds all but 5 per cent of the values. When percentiles are estimated by ranking the items of a finite sample, the percentile generally falls between two of the observed values, and the midway value is often taken.
The terms tercile, quartile, quintile and decile should refer to the percentiles which divide the distribution into 3, 4, 5, or 10 equal parts, respectively. These terms are, however, often used to signify a specified fraction of the total number of ranked values. Thus the value of a particular element may be predicted to be in a specified tercile or quintile of the appropriate distribution.