Douglas Harper's Etymology Dictionary
"pertaining to measure, measurable," c.1600, from Medieval Latin mensuralis, from Latin mensura "a measuring, measurement" (see measure (v.)).
a. 1 Of or pertaining to measure or measurement. 2 (context music English) Having a fixed rhythm.
The Collaborative International Dictionary
Mensural \Men"su*ral\, a. [L. mensuralis.] Of or pertaining to measure.
adj. of or relating to measure
having notes of fixed rhythmic value