A diesis is generally used to mean the difference between three justly tuned major thirds and one octave.
From the en.wikipedia.org
The use of tyronic letters clearly shows, that the enharmonic diesis was used as a kind of melodic attraction within the diatonic genus, which sharpened the semitonium.
From the en.wikipedia.org
Even in Guido of Arezzo's treatise Micrologus, at least in earlier copies, there is still a passage which explains, how the diesis can be found on the monochord.