A digital master is an image, PDF file, digital recording or another digital asset preserved as the "original" for the purpose of archival storage, reuse and re-expression. For images, it is the digital analogue to a photographic negative.
As the master from which variations for specific uses can be derived, the digital master may be in the form of its initial capture (like an unretouched photograph) or in a form that has been somehow enhanced, reformatted or edited (like a manipulated photo or a completed film).