The following is from Harvard Dictionary of Music 2nd Edit.

From the definition of 'Discant' (bold added):

[...] The ideal of contrary motion was mentioned in the 13th-century "Tractatus de Discantu" (in J.-A.-L. de la Fage, Essais, i, 358; CS i, 311) but became an essential characteristic of discant theory only at the end of the 13th century, when modal rhythm was replaced by mensural rhythm.

Full page of context:

I've never heard these two terms: mensural and modal rhythm. Google gave nothing, neither did Britannica.

What do these two terms mean?