Mezzotint is the oldest purely tonal intaglio printmaking technique, invented by Ludwig von Siegen in 1642. The name stems from "mezzo-tinto," which means "half-tone." It is a direct, non-acid technique, simple in theory, but notoriously difficult to execute well.
In a nutshell, you roughen the surface of a copper plate with a tool called a rocker. This creates a surface that will print totally black when inked and wiped like an etching or engraving. You then scrape and burnish this textured plate to gradually reveal lighter tones.