In spaceflight, a plasma sheath is an envelope of ionized gas, or plasma, that surrounds a body moving through an atmosphere at hypersonic velocity. A plasma sheath forms around a spacecraft during reentry and interrupts or interferes with communication with the ground.
More generally, a plasma sheath is the boundary layer of charged particles between a plasma and its surrounding walls, electrodes, or other plasmas. The sheath is generated by the interaction of the plasma with the boundary material. Current flow may be in only one direction across the sheath (single sheath), in both directions across the sheath (double sheath), or when the plasma is immersed in a magnetic field, it may flow along the sheath surface at right angles to the magnetic field (magnetic current sheath).