What are Epics in regard of features and backlog items in Team Foundation? [closed]

From this link I assert that a feature is a set of backlog items, but what is an epic supposed to be?

The general consensus is that:

• Product backlog Item is something that can be delivered in a single sprint.
• Feature is something that can't be delivered in a single sprint, but that can be delivered in a single release.
• Epic is something that transcends releases.
• Theme is a cross cutting concern.

Theme is generally implemented as a tag in TFS and VSTS.

This is a practice that is in line with Scrum Framework, Nexus Framework, and SAfE methodology.

From this link:

"Epics are hierarchically above Features. Features are mapped to Epics, like Backlog items are mapped to Features"