Galvanic isolation
Two circuits that exchange information or power with no conductive path between them — coupled only through light (optocoupler), magnetic field (transformer), or capacitance (isolator ICs). The barrier is for safety, noise, or breaking ground loops.
In practice
The barrier is only as real as its weakest accidental bridge — and the classic accident is a shared ground through a debug connector or mounting hole. Isolation isn't a part you place; it's a property you must verify across every conductive path.