A large user story that awaits decomposition into smaller stories prior to implementation. Epics are typically stories that are far off on the development horizon, usually lower priority items. When an epic story works its way up the backlog, it is usually decomposed into smaller stories.
NB: One can easily be tempted to associate the term ‘epic’ with importance; in Agile, epic relates only to size.
Mike Cohn coined the term epic as it relates to Agile.