A measure of how easily a material allows a magnetic field to pass through and build up inside it (think of it like magnetic conductivity). Air and vacuum have the lowest permeability (baseline). Materials like ferrite and iron have much higher permeability, meaning they concentrate and amplify the magnetic field passing through them. This is why adding a ferrite core inside a coil can multiply its field strength many times over for the same amount of current. Permeability is expressed as a ratio relative to air (µr), where air equals 1 and common ferrite materials range from 10 to 500.
A magnetic core turns coil current into field, but not without limit. The curve rises steeply at first, and that slope is the permeability, how readily the material carries and amplifies the field. As the current climbs, the curve bends over to a flat plateau: every magnetic domain in the core is aligned and no more field can be gained. Past that point extra current only heats the coil, which is why core material and drive level are chosen to stay below saturation.
