A persistent identifier (PI or PID) is a long-lasting reference to a document, file, web page, or other object.
The term "persistent identifier" is usually used in the context of digital objects that are accessible over the Internet. Typically, such an identifier is not only persistent but actionable: you can plug it into a web browser and be taken to the identified source.
Of course, the issue of persistent identification predates the Internet. Over centuries, writers and scholars developed standards for citation of paper-based documents so that readers could reliably and efficiently find a source that a writer mentioned in a footnote or bibliography. After the Internet started to become an important source of information in the 1990s, the issue of citation standards became important in the online world as well. Studies have shown that within a few years of being cited, a significant percentage of web addresses go "dead," a process often called link rot. Using a persistent identifier can slow or stop this process.
An important aspect of persistent identifiers is that "persistence is purely a matter of service." That means that persistent identifiers are only persistent to the degree that someone commits to resolving them for users. No identifier can be inherently persistent.
Persistent identifiers are often created within institutionally administered systems. These include: Archival Resource Keys (ARKs), Digital Object Identifiers (DOIs), the Handle System, Persistent Uniform Resource Locators (PURLs), Uniform Resource Names (URNs), and Extensible Resource Identifiers (XRIs). However, some regular URLs (i.e. web addresses), maintained by the website owner, are intended to be long-lasting; these are often called permalinks.