mastic, resin obtained from the small mastic tree Pistacia lentiscus (of the sumac family), found chiefly in Mediterranean countries. When the bark of the tree is injured, the resin exudes in drops. It is transparent and pale yellow to green in color. Mastic is used chiefly in making varnish but is also used medicinally as an astringent and, with aniseed, to flavor a distilled liquor called mastic. The term mastic is also applied to certain caulking and adhesive compounds, especially those consisting of a mineral filler, a resinous binder (e.g., asphalt), and a volatile solvent.