The point where a magnetic core material can't get any more magnetized no matter how much more current you push through the coil. Beyond this point, increasing current produces diminishing returns in field strength. This is why core material selection matters. A saturated core wastes energy as heat instead of producing stronger field. All magnetic domains in the material are aligned and no further magnetization is possible.
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.
