(surface-shape around a black hole through which light cannot escape)
A black hole's event horizon is the closed surface-shape
surrounding the black hole consisting of a boundary from within which
electromagnetic radiation (EMR) cannot escape. Current physics says there cannot be a way
to find out what is within the event horizon other than the black
hole's mass, rotation, and charge. The event horizon can be
thought of as the outer surface of the black hole, a useful presumption
for some approximate models, but the weirdness of black holes results
in caveats.
For the simplest black hole (no rotation or charge), the event
horizon is a spherical shell-shape located at the
Schwarzschild radius (2GM/c²) from the black hole's center.
Any black hole rotation makes it "wider" (oblate, as does the
rotation of a planet or star), but likely doesn't change the
order-of-magnitude.