Inductor equivalent of capacitor's charge
Magnetic flux is the complement of charge. 

Just as a capacitor is defined by the relationship \$Q = CV\$, an inductor is defined by the relationship \$\varphi=LI\$, where \$\varphi\$ is the magnetic flux. 

Just as the capacitor formula becomes \$I = \dfrac{dQ}{dt} = C\dfrac{dV}{dt}\$ when we look at time variation, the inductor formula becomes \$V = \dfrac{d\varphi}{dt} = L\dfrac{dI}{dt}\$.

Just as we can generalize the idea of a capacitor to the nonlinear case with the relationship \$f(Q,V)=0\$ we can generalize the idea of an inductor with the relationship \$f(\varphi,I)=0\$.