In the magnetosphere, the core of the magnetotail in which the plasma is hotter and denser than in the tail lobes north and south of it. The plasmasheet is thought to be separated from the tail lobes
by the sheet of the ”last closed field lines” and it typically lies beyond geosynchronous orbit.
More or less extensive ice-covered area of relatively high and uniform elevation, which may include one or more domes and be limited by mountain walls or not so limited, cf. plain, snowfield.
The location where two plates meet is called a plate boundary. it is often a broad zone where the plate interactions are not well understood. [Wikipedia]