What is the defining characteristic of a black hole?

The defining characteristic of a black hole is that it is a region of space resulting from the collapse of a massive star. When a star exhausts its nuclear fuel, it can no longer support itself against gravitational forces and undergoes a gravitational collapse. If the remaining core mass exceeds a certain limit, known as the Schwarzschild radius, it continues to collapse into an infinitely dense point called a singularity, surrounded by an event horizon. The event horizon is the boundary beyond which nothing, not even light, can escape the gravitational pull of the black hole, which is why black holes are often described as "black."

The other options do not accurately describe black holes. Bright light emission typically describes stars and other celestial bodies, but black holes themselves do not emit light; instead, they can be identified by the effects of their gravity on surrounding objects and matter. The notion of a large mass that cannot be observed aligns more closely with the effects of black holes as inferred through indirect measurements, but it does not capture their formation process. Lastly, a large star still in the process of forming does not describe a black hole, as black holes arise from the death of massive stars, rather than their formation stage.