Mensural notation
(from the Collins dictionary)

The term is most famously used to describe Mensural notation, a European system of musical notation developed in the late 13th century. Before this era, medieval plainchant/Gregorian chant used a system without strictly measured rhythms. Mensural notation introduced the ability to write out the exact duration of each note (such as a longa or brevis) using numerical proportions, forever changing the landscape of polyphonic music.