Source : Webster's Revised Unabridged Dictionary (1913)

Sesquitertian \Ses`qui*ter"tian\, Sesquitertianal
\Ses`qui*ter"tian*al\, a. [Sesqui- + L. tertianus belonging to
   the third. Cf. {Tertian}.] (Math.)
   Having the ratio of one and one third to one (as 4 : 3).