An epic is a large body of work, too big for one sprint, that is broken down into smaller user stories or tasks.
Epics group related work around a goal, for example "New checkout" or "Single sign on". They sit between high level initiatives on a roadmap and the individual stories a team works on.
Example
The epic "Customer notifications" might contain stories for email receipts, push notifications and a notification settings page, each with its own sub-tasks.
Epics in Starlex
Group issues under epics, see progress roll up, and plan epics on the Gantt chart, which groups by epic by default.