You may wish to compare Defritum, Caroenum, Sapa and Wine Syrup.
To make your own: Crush grapes. Put crushed grapes and their juice in a bowl, let sit uncovered for 24 hours. Strain and discard all the solids. Simmer until it has thickened to a syrup (this may take several hours), then bottle and refrigerate.
Refrigerate purchased bottles after opening.
Sapa was made by the Romans. Romans disagreed on how much reduction of volume was needed; some said you had to reduce the grape juice to 1/3 of its original volume, others said reducing it to 1/2 was enough to call it Sapa.
Columella says “sapa” was reduced by one-half: “Some people boil their must down by a quarter, that they have have put in lead vessels, others cook off a third of it, but without a doubt, if you cook of a half of it, a better sapa will be made.” (“Quidam partem quartam eius musti, quod in vasa plumbea coniecerunt, nonnulli tertiam decoquunt; nec dubium quin, ad dimidiam si quis excoxerit, meliorem sapam facturus. De Re Rustica, Chapter 12, 19,1)
They often used Sapa along with verjuice to create a sweet-sour taste.
Also spelt “Saba” and “Sabba.”