An armature coil, also known as a rotor coil, refers to a coil of wire wound around the armature core of an electric motor or generator. It is a crucial component that interacts with a magnetic field to produce mechanical motion or electrical energy. The armature coil is typically made of copper or aluminum wire and is responsible for generating the electromagnetic forces necessary for the motor or generator's operation. It plays a vital role in converting electrical energy into mechanical energy or vice versa.