magnetic permeability

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The degree of magnetization of a substance in a given magnetic field, equal to the ratio of magnetic flux density to magnetic field strength.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics a measure of the magnetisation of a material in a magnetic field; symbol μ. Materials with values less than 1 are diamagnetic those above are paramagnetic.