quartile
n : (statistics) any of three points that divide an ordered
distribution into four parts each containing one quarter
of the scores


Source: Webster's Revised Unabridged Dictionary (1913)

Quartile \Quar"tile\, n. [F. quartile aspect, fr. L. quartus the
fourth. See Quart.] (Astrol.)
Same as Quadrate.


I found severalsites on Internet that used "tertile" to mean data divided by two points into three groups, but was
unable to find a dictionary definition.