You can definitely have an epic within an epic. Call it whatever you will, just make sure that all stakeholders in the project have a clear understanding of whatever hierarchy & definitions you choose to use and what each item is intended to do on the project.
Personally I'm a fan of keeping things as simple as possible and whenever I can, advocate for a two level hierarchy. Epic>Story aka Intangible>Tangible
Epic is anything that is too big to be story point estimated; it extends beyond the tactical time-frame of an iteration and the immediate consideration of the delivery team. A story is something a team can commit to delivering to the customer by the end of the iteration.
As an aside, your main metric (velocity) is based around the story. By keeping the hierarchy as flat as possible and keeping your buckets simple you also reduce the risk of stakeholders trying to use things like tasks, features, epics, milestones, themes, etc to drive a delivery schedule (try to control the cone of uncertainty), measure individual productivity, derive story point to hour conversions, and a handful of other waterfallish-agile things that organizations with lots of management levels love to engage in.