A transformer with a 1:1 winding ratio to provide an isolated mains voltage supply..
An isolation transformer is used to power a single item of equipment under test. The supply from the transformer is not referenced to earth/ground, and therefore contact with either of the output terminals will not result in an electric shock.
Although this provides some additional safety, note that contect with both terminals will result in a shock, so this device is suitable for use in a testing laboratory environment only.
Another use of this type of transformer is to avoid accidental faults to ground when using mains powered test equipment such as oscilloscopes with other mains powered items.