Digital Object Identifier (DOI)
DOI is both a unique identifier of a piece of digital content AND a system to access that content digitally. Persistent object identifier.
‘The ISBN for the 21st Century’ -- Norman Paskin.
DOI system has two main parts: (the identifier and a directory system) and a third logical component, a database.
Developed by AAP (Association of American Publishers), now managed by International DOI Foundation.