A Persistent Unique Identifier (PID) is a string of letters and numbers used to distinguish between and locate different objects, people, or concepts. A well-known example of a PID is a Digital Object Identifier (DOI) which is used to locate specific digital objects, frequently a journal article. Another example is ORCiD, a PID for researchers.

PIDs can be created by an institution that maintains a collection of resources. For example, the National Library of Medicine’s Medical Subject Headings (MeSH) assigns a Unique ID to each term. Alternatively, a third party can create and track PIDs across many institutions, each of which can then assign PIDs to resources, objects, or other concepts. For example, DOI’s are maintained by the International DOI Foundation (IDF). The IDF works with various organizations to register new digital objects and to ensure that no two objects receive the same DOI. The organizations that manage registration include DataCite and CrossRef.