A process where a metal plate is altered to guide ink onto a pressed paper – specifically the image area is below the flat surface of the image surface or plate. It is the sunken part that hold the ink, therefore the positive part of the design, and so makes it an ‘Intaglio’ process.
See MoMA video: http://www.moma.org/explore/multimedia/videos/151/939
Etching can be done in an acid-resistant ground that covers a metal (copper or zinc) plate. Where the ground is removed, the acid-resist is removed, so when bathed in an acid bath the metal is etched by the acid to replicated the scraped image in the metal plate.
Engraving Direct cutting away of a surface to create the incised ink wells.