Epic
A large body of work too big to deliver in one iteration, so it's broken into multiple user stories that ship across several sprints. An epic groups related stories under a shared outcome — 'checkout redesign', 'SSO support' — and is tracked to completion as those stories close, giving stakeholders a unit of progress larger than a story but smaller than a release.
Epics sit in the middle of the planning hierarchy: themes and initiatives above, stories and tasks below. The discipline is decomposition — an epic that never gets split into sprint-sized stories becomes a black box nobody can forecast. Good teams write a one-line epic outcome and a rough list of stories, then refine just-in-time rather than fully specifying the whole thing up front.