The Digital Object Identifier (DOI) System is a managed system for persistent identification of content-related entities on digital networks. These entities may be content items (digital files, physical objects, abstract works), or any related entities in a content transaction (e.g. licenses, parties, etc.). "DOI" is sometimes used to mean the identifiers within this system; hence the use of the term alone is deprecated unless the meaning is sufficiently clear from an earlier mention or the specific context: instead it should always be used in conjunction with a specific noun. The DOI name is the identifier string that specifies a unique object (the referent) within the DOI System; the DOI syntax is the form and sequence of characters comprising any DOI name, specifically the prefix element, separator, and suffix element; and the DOI System is the functional deployment of DOI names as identifiers in computer sensible form through assignment, resolution, referent description, administration, etc.

The DOI System can be used to identify physical, digital, or abstract entities; these names resolve to data specified by the registrant, and use an extensible metadata model to associate descriptive and other elements of data with the DOI Name. The DOI System is an implementation of the Handle System and of the indecs Content Model and so inherits the design principles and features of each.

The DOI System is implemented through a federation of DOI Registration Agencies, under policies and common infrastructure provided by the International DOI Foundation, which developed and controls the system. The DOI System has been developed and implemented in a range of publishing applications since 2000; by early 2009 approximately 40 million DOI names had been assigned.