All delivered Tactical Product Backlog items in a stage.
The Stage Increment is the sum of all Tactical Product Backlog-items, that are delivered in a Stage – according the Joint Definition of Done. It is a refined and production-ready delivery by the development team at the end of Stage (and at the end of each Sprint) . In case of a service team, it is the increment of delivered services.
In Scaled Agile Framework (SAFe) the Stage Increment is called a System Increment.