When changing current in one coil induces an EMF in the other, the phenomenon is called mutual induction. The strength of the EMF induced depends on the mutual inductance of the pair of coils.

.Since the rate of change of flux linking with the coil, B depends upon the rate of change of current in the coil A.
The rate of change of current in coil A is directly proportional to the magnitude of the mutually induced emf.

M is called the Constant of Proportionality and is also called as Mutual Inductance or Coefficient of Mutual Inductance.