could some one explain the term event horizon ? What does this actually refer to and who coined the term ?

Answer 1

The event horizon is the name given to the boundary between two regions around a black hole. Outside the event horizon, the gravity is weak enough that light traveling away from the black hole actually makes progress and can escape. Inside the event horizon, the gravity is so strong that light cannot escape. Even if it's traveling directly away from the center of the black hole, it will still be drawn closer to the center.

The event horizon is spherical in shape.

Answer 2

The horizon (from Greek orizein, to limit) is the line that separates earth from sky. More precisely, it is the line that divides all of the directions one can possibly look into two categories: those which intersect the Earth, and those which do not. At many locations, the true horizon is obscured by trees, buildings, mountains and so forth. The resulting intersection of earth and sky is instead known as the visible horizon.