What is the name of the formula linking a black hole’s entropy to the area of its event horizon?

The formula linking a black hole’s entropy to the area of its event horizon is the Bekenstein–Hawking formula.

Jacob Bekenstein proposed in the 1970s that a black hole should possess entropy proportional to its horizon area. Stephen Hawking’s calculation of black-hole radiation supplied the temperature needed to establish the relationship quantitatively. In modern units, the entropy equals one quarter of the horizon area measured in Planck units.

This result is remarkable because ordinary systems usually have entropy that scales with volume, while black-hole entropy scales with surface area. That observation became a major clue for the holographic principle, which suggests that information in a volume may be represented on its boundary. The formula remains one of the deepest links between gravity, thermodynamics, and quantum theory.