Mutual Inductance : The mutual inductance, of a pair of coils, equals the magnetic flux linked with one of them due to a unit current in the other.
Alternatively,
The mutual inductance, of a pair of coils, equals the emf induced in one of them when the rate of change of current in the other is unity.
Factors affecting the mutual inductance of a pair of coils :
(i) The sizes of the two coils
(ii) The shape of the two coils
(iii) The distance of separation between the two coils
(iv) The nature of the medium between the two coils.
(v) The relative orientation of the two coils.