Reluctance in a magnetic circuit is defined by the formula involving length, permeability, and cross-sectional area. It remains largely constant with respect to magnetic flux, as length and cross-sectional area do not change under typical conditions. The primary variable affecting reluctance is permeability, which is constant for free space and similar for most materials, except for ferromagnetic materials that exhibit a BH saturation curve. The discussion highlights that magnetorestriction may introduce minor variations in specific cases. Understanding these principles is essential for analyzing magnetic circuits effectively.