Product Backlog Item any item that is one the Backlog list, which will include User Stories, Epics and possibly technical stories to deal with technical debt, etc. Backlog items should have priorities assigned to them by the Product Owner in accordance to his conversation with the Stakeholders. It makes sense that the main criteria of the prioritization are ordered by:
Needs (for Minimum Viable Product)
Wants
Shoulds
Coulds
Product Backlog Items should fulfill the INVEST criteria.