Black Hole
A black hole is a region of space where gravity is so strong that nothing, not even light, can escape once it crosses the event horizon.
Definition
A black hole is a place in space where gravity pulls so hard that nothing can escape, not even light, which is why it looks black; the biggest stars leave one behind when they collapse. Its boundary is the event horizon, inside which the escape velocity exceeds the speed of light. Stellar black holes form from stars more than about $20$ times the Sun's mass, while supermassive black holes of millions to billions of solar masses sit at the centers of most large galaxies. In general relativity a black hole is a region of spacetime curved so strongly that a horizon forms, and for a non-rotating mass $M$ its radius is the Schwarzschild radius $r_s = 2GM/c^2$. Black holes are fully described by mass, spin, and charge, and they are detected through accretion disk X-rays, the orbits of companions, gravitational lensing, and gravitational waves from mergers.
Example
In 2019 astronomers released the first image of a black hole's shadow, in the galaxy M87, a dark circle ringed by hot glowing gas. Sagittarius A*, at the center of the Milky Way, has about $4$ million solar masses, measured by tracking stars that orbit it at thousands of kilometers per second. For the Sun, $r_s \approx 3\ \text{km}$; for Earth it is about $9\ \text{mm}$; and for Sagittarius A* it is about $1.2 \times 10^7\ \text{km}$, roughly $17$ times the Sun's radius.
Key Insight
A black hole is not a cosmic vacuum cleaner: if the Sun became a black hole of the same mass, Earth would keep orbiting exactly as it does now, just cold and dark. We never see the hole itself, only its effects, such as stars orbiting an invisible mass or gas heated to millions of degrees as it spirals in. At a distance, a black hole's gravity is ordinary gravity for its mass; what makes it special is compactness, and the 2015 LIGO detection of merging black holes confirmed these objects in the most direct way possible.