Encoded Archival Description
Encoded Archival Description (EAD) was defined in the EAD Application Guidelines for Version 1.0 as:
... a data structure standard for preserving the hierarchy and designating the content of descriptive guides to archival holdings worldwide. It enables Internet delivery of these guides and also ensures their permanence by providing a stable, non-proprietary data storage environment from which data can be transferred to other software environments as necessary. In technical terms, EAD comprises a Document Type Definition (DTD) for encoding archival finding aids that is written following the syntactic rules of Standard Generalized Markup Language (SGML) and Extensible Markup Language (XML).
The international EAD standard for encoding electronic versions of archival finding aids is jointly maintained by the Library of Congress and the Society of American Archivists (SAA). The SAA sums it up nicely on their EAD Help Pages.
The set of guidelines developed for local practice is available online within The University at Buffalo Best Practice Guidelines for Encoded Archival Description.