See Transformer. An isolation transformer has an equal number of windings between the input and output coils. The output voltage is therefore the same as the input voltage. Since magnetic fields are relatively slow to change, an isolation transformer can protect circuitry from damaging electrical noise.
A transformer with a 1:1 (one-to-one) power ratio. That is, the signal level going in equals the signal level coming out. Commonly seen used in direct boxes or microphone line splitters. This allows the same signal to drive different inputs such that they two inputs don't interfere with each other, but are isolated from each other.
A transformer in which one or more output windings is electrically separated from the input winding and all other output windings by an insulation at least equivalent to double insulation or reinforced insulation.