Balsam is the resin that leaks out of trees when they are cut. Several plants with a fragrant resin are referred to as balsam. Balsam, oak, mulberry, and terebinth are not even superficially similar and are unrelated.
Resin was added to wine, as we learn from ancient writers, as a preservative and for medical benefits. It was even used to coat the interior of ceramic vessels, so the wine would not seep out. The Greek historian Plutarch (±46-120CE.) observed that resin was used to enhance the bouquet; it improved the taste of wine by Greeks on Euboea, Italians along the Po River and vintners around Vienne in the Rhône Valley.
Pliny the Elder, the famous 1st century A.D. Roman scientist, devoted a good part of one of his books ("Natural History") to the problem of preventing wine from turning to vinegar. Tree resins - pine, cedar, and often terebinth (which Pliny described as the "best and most elegant" resin) - were added to Roman wines for just this purpose. Roman also used resins for medicinal purposes; indeed, modern chemical investigations have proven that resins can kill certain bacteria, thereby protecting organic compounds from degradation.
Analyses of residues in wine amphorae from ancient shipwrecks and storage jars excavated on land have made it possible in recent years to trace the trade and consumption of resinated wines through antiquity to the dawn of winemaking in the Neolithic Near East. The earliest evidence, reported in 1997 by Patrick McGovern and his colleagues at the University of Pennsylvania, comes from Hajji Firuz Tepe in Iran, where wine mixed with terebinth resin was stored in vessels dating to between 5.400 and 5.000 B.C. Those of us who drink retsina, therefore, are following one of the oldest winemaking traditions in the world.