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.
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.
Studies have shown that within a few years of being cited, a significant percentage of web addresses go "dead",[2][3] a process often called link rot.
However, some regular URLs (i.e. web addresses), maintained by the website owner, are intended to be long-lasting; these are often called permalinks.