Transformer

Isolation Transformers which often has symmetrical windings is used to decouple two circuits. An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers can be used to block transmission of DC signals from one circuit to the other, but allow AC signals to pass. They will also block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments.

Isolation Transformers have primary and secondary windings that are kept physically separate. Sometimes isolation transformers are called "insulated transformers". This is because the primary and secondary windings are insulated from each other.
A 3 phase transformer, there is a three-legged iron core as shown below. Each leg has a respective primary and secondary winding.  Thus a 3 phase isolation transformers is a 3 phase transformer which has isolated primary and secondary windings to allow the power input to be isolated from the power output.