Sapa, in Latin, or Siraion (σίραιον), or Hepsema (έψημα), in Greek, is a grape syrup made from grape juice by boiling it and reducing it to 1/3 of its original volume. Many Greek and Roman authors mention it by name, including Hippocrates in the 5th cenutry BCE, but only Pliny the Elder (1st century CE) describes how it was made.
nam siraeum, quod alii hepsema, nostri sapam appellant, ingenii, non naturae, opus est musto usque ad tertiam mensurae decocto. quod ubi factum ad dimidiam est, defrutum vocatur.
Siraeum, by some called hepsema and in our country sapa, is a product of art, not of nature, made by boiling down must to a third of its quantity; must boiled down to only one-half is called defrutum.
Latin text and English translation from Harris Rackham. Pliny: Natural History; with an English Translation in Ten Volumes. Volume IV. Book XIV, Chapter 11, Paragraph 80. Loeb Classical Library. Harvard University Press, 1945. pp. 240-241.
Sapa was commonly used as a sweetener in many Greek and Roman recipes. Due to the reduction, it is sweeter than the original grape juice it was made from.