Return electrode: A large pad attached to the patient’s skin through which electrical energy is returned to the electrosurgical unit.

It is a capacitive pad, meaning it relies on the patient contact area. If there is a small patient area in "contact" with the pad, current flow is automatically decreased to a safe level.

Return electrode monitoring (REM): A technology incorporating a "split" conductive surface patient return electrode

The purpose of all patient return electrodes during electrosurgery is to safely exit current from the patient with minimal resistance and without heat build-up.

というわけで、patient return electrode とは、患者に接触しているアースパッドのことでした。電気手術中、患者を保護するために使っています。