PISA (Persistent Identification and Sustainable Access) is a standard using PIDs (Persistent Identifier) for referencing and citing documents, files and language resources (e.g digital dictionaries, text corpora, linguistic annotated corpora). PID is an electronic identification referring to or citing electronic documents, files, resources, resource collections such as books, articles, papers, images etc. In contrast to URL, which can be changed if the resources move to other servers or to other organisations, PID does not change and always remains the same. That allows the necessary resources to be reliably identifiable and to remain accessible at all times.

There are different kinds of PID such as Uniform Resource Name (URN), the persistent URL (PURL), the digital object identifier (DOI), National Bibliography Numbers (NBNs), etc.

The PISA standard does not describe PID itself, but it describes the requirements for PIDs (i.e what should be respected) and offers a basic approach to effectively using PIDs for referencing and citing language resources various formats.