What term describes the dark central region seen in the Event Horizon Telescope image of M87*?

The dark central region seen in the Event Horizon Telescope image of M87* is called the black-hole shadow.

A black-hole shadow is the apparent dark region created when a black hole captures light or bends it away from an observer. It is larger than the event horizon because photons passing near the black hole can be strongly deflected before either escaping or being captured.

In the 2019 M87* image, the shadow appeared inside a bright ring of emission from hot plasma near the black hole. The ring’s brightness is uneven because the plasma is moving at relativistic speeds and the black hole’s gravity magnifies some parts of the emission.

The shadow is not a solid surface and should not be confused with the event horizon. The event horizon is a causal boundary, while the shadow is an image-forming optical effect shaped by gravity, light paths, and the surrounding emitting material.