Differential capacitance in physics, electronics, and electrochemistry is a measure of the voltage-dependent capacitance of a nonlinear capacitor, such as an electrical double layer or a semiconductor diode.
[1][2] In electrochemistry differential capacitance is a parameter introduced for characterizing electrical double layers: where σ is surface charge and ψ is electric surface potential.
However, the term is meaningful when applied to any two conducting bodies such as spheres, and not necessarily ones of the same size, for example, the elevated terminals of a Tesla wireless system and the earth.
[3] "The differential capacitance between the spheres is obtained by assuming opposite charges ±q on them ..."[4]Another form of differential capacitance refers to single isolated conducting bodies.
The definition of the absolute potential of the body depends on what is selected as a reference.