An intaglio etching technique in which a metal plate is sprinkled with a finely-powdered resin (asphaltum, rosin, etc.); the plate is heated to melt the resin, then cooled, and placed in an acid bath. The acid lightly etches areas not covered with the resin. The results in a plate with fine pockmarks. The process was invented in the 1760s by J.B. LePrince. Aquatints were popular until the late 1830s.

Source: Art & Architecture Thesaurus

http://vocab.getty.edu/page/aat/300053242