Mu is the AC gain of a tube with the plate current held constant. In a perfect tube, mu would always be the same.
What does Z mean in impedance?
Impedance, denoted Z, is an expression of the opposition that an electronic component, circuit, or system offers to alternating and/or direct electric current. Impedance is a vector (two-dimensional)quantity consisting of two independent scalar (one-dimensional) phenomena: resistance and reactance.