When magnetic flux produced by one coil induces an EMF in a second coil, what is this phenomenon called?

The phenomenon where magnetic flux produced by one coil induces an electromotive force (EMF) in a second coil is known as mutual inductance. This occurs when the change in current in the first coil generates a changing magnetic field, which subsequently influences or induces a voltage in the second coil. Mutual inductance is an essential principle in various electrical and electronic applications, such as transformers, where energy is transferred from one coil to another through the magnetic field.

Inductive reactance, self-inductance, and electromagnetic induction are related concepts but do not specifically describe the interaction between two coils. Inductive reactance pertains to the opposition to alternating current by inductors, self-inductance refers to the induction of EMF in the same coil due to its own changing current, and electromagnetic induction is a broader term that includes both mutual and self-induction. Thus, mutual inductance is the appropriate term for the scenario described in the question.