Final answer:
The event horizon is the defining boundary of a black hole, separating its inescapable gravitational pull from the rest of the universe.
Step-by-step explanation:
The property that separates a black hole from the rest of the universe is the event horizon. This boundary represents the point beyond which nothing, not even light, can escape the gravitational pull of the black hole. The radius of this critical surface is known as the Schwarzschild radius, and it is directly proportional to the mass of the black hole.
Within the event horizon lies the singularity, a point where density is thought to be infinite and the laws of physics as we know them no longer apply. However, for a black hole to affect its surroundings, the defining characteristic is its event horizon. Once a particle crosses this threshold, it cannot escape or communicate with the rest of the universe.