Flux tube
A flux tube is a region of space bounded by a flux surface, i.e., a surface such that the magnetic field is everywhere perpendicular to the surface normal.
In flux coordinates, such a surface has cylindrical topology. In a closed magnetic field region, the topology is toroidal.
The magnetic flux traversing any cross sectional area of a flux tube is invariant.
Contrary to magnetic islands, that are bounded by a separatrix, there is not necessarily any essential dynamical difference between the regions inside and outside of a flux tube.