Source : Webster's Revised Unabridged Dictionary (1913)

Trisect \Tri*sect"\, v. t. [imp. & p. p. {Trisected}; p. pr. &
   vb. n. {Trisecting}.] [Pref. tri- + L. sectus, p. p. of
   secare to cut. See {Section}.]
   1. To cut or divide into three parts.

   2. (Geom.) To cut or divide into three equal parts.

Source : WordNet®

trisect
     v : cut in three; "trisect a line"