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An epic captures a large body of work. It is essentially a large user story that can be broken down into a number of smaller stories. It may take several sprints to complete an epic. 

An epic can span more than one project, if multiple projects are included in the board to which the epic belongs.

(info) Epics only apply to Scrum boards.

Go to Plan mode to see a list of epics at the left of the screen. Drag-and-drop epics to re-order them. Click an epic's name to view all the issues belonging to that epic, across all sprints:

Screenshot: viewing an epic in Plan mode

You can also view an epic issue in JIRA to see a list of the stories (issues) it contains:

Screenshot: viewing an epic in JIRA

(tick) You can also use the JQL Epic Link  field to search for an epic's issues — see the JIRA JQL documentation for details.