Isolation
Separating a subject from its surroundings so it reads alone, using depth of field, framing, tone or timing.
In depth
Isolation is a broader idea than shallow focus: a subject can be isolated by a pool of light, by an empty background, by being the only sharp thing in a moving crowd, or simply by waiting until nothing else is nearby. The technique chosen changes the meaning as much as the appearance.
Examples in practice
A commuter caught in a shaft of light while the rest of the concourse falls dark is isolated without any focus trick.