The following is from Harvard Dict of Music 2nd Edit. (from the definition of 'Discant')

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

What do these two terms mean?

The Text: '(...) 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 rhytm was replaced by mensural rhytm.'