What is the term that defines the ability of a magnetic material to oppose magnetic flux?

The term that defines the ability of a magnetic material to oppose magnetic flux is reluctance. Reluctance is essentially the magnetic equivalent of resistance in electrical circuits. It quantifies how much a material resists the passage of magnetic field lines, similar to how resistance quantifies opposition to the flow of electric current.

In magnetic circuits, materials with high reluctance will impede magnetic flux more than those with low reluctance. This concept is crucial when designing magnetic systems, such as transformers and inductors, where understanding how materials interact with magnetic fields can affect performance and efficiency.

The other terms—capacitance, inductance, and permittivity—relate to different aspects of electrical and magnetic behavior but do not specifically address the opposition to magnetic flux. Capacitance pertains to the ability of a system to store charge, inductance relates to the ability to store energy in a magnetic field due to current flow, and permittivity is concerned with how electric fields interact with materials. Thus, for the context of magnetic materials and their opposition to flux, reluctance is the appropriate term.