Magnetic island
A magnetic island is a region of space without magnetic field sources where no field lines enter or leave. Therefore, it is bounded by a separatrix, isolating it from the rest of space. Its topology is toroidal of necessity, but its winding number can be different from 1.
In the context of magnetic confinement fusion, the basic magnetic field configuration consists of toroidally nested flux surfaces, while each flux surface is characterised by a certain value of the magnetic shear. Magnetic islands can appear at flux surfaces with a rational value of the safety factor q = m/n. [1] The rupture of the original topology of toroidally nested flux surfaces needed to produce the island requires the reconnection of magnetic field lines, which can only occur with finite resistivity.