What term is used for a material having a permeability less than that of a vacuum?

The term used for a material that has a permeability less than that of a vacuum is "diamagnetic." Diamagnetic materials are characterized by their weak repulsion to magnetic fields and an induced negative magnetic susceptibility. When exposed to an external magnetic field, these materials do not retain any magnetic properties once the field is removed, distinguishing them from ferromagnetic and paramagnetic materials, which can either retain magnetism or respond more strongly to magnetic fields.

In contrast, ferromagnetic materials exhibit strong magnetic properties and can retain magnetism, while paramagnetic materials have a positive, but weak, response to magnetic fields and do not retain magnetism once the external field is removed. The term "magnetic" can be broad and encompass various types of magnetic materials, but specifically refers to those that show magnetic properties, which does not apply to materials having a permeability less than that of a vacuum.