About this term
Source: Oxford University Press
Impression of a print taken from a plate or block, usually before the production of the regular edition. Traditionally, it was often done (as a ‘trial’) in order to see how the work was progressing. Sometimes the proof is corrected (‘touched’) by hand by the artist. Printed ‘proof’ editions of the finished state of the print, but before the addition of lettering or other details, are common from the 18th century.
From Grove Art Online