What exactly is the event horizon of a black hole?

Event horizon is define as the point of NO return, a surface covering the black hole beyond which no kind of information from the inside is able to escape. This includes light. Hence if you look at the black hole from infinity, you will see the black surface as the actual even horizon. However, if we keep moving towards it, the black surface will also appear to become smaller. Hence one cannot actually know if he/she has crossed the even horizon.

If a black hole could exist the radius of the event horizon would depend on it's mass. A minimum size black hole would have a diameter of about 3 km. and if it could be seen you would have to be dangerously close as it would have a mass of about 2 of our suns.. The quantum effect would mandate that the thickness of the one way membrane could be no more than one-tenth of a mm thick,no matter the radius of the entity. Unless a photon was emitted from the surface the black hole would remain invisible.

"Or is the event horizon a fixed point (depending on the mass of the Black Hole) that is the true point of no return from which even light cannot escape?"

It's that.

It's the point where the escape velocity equals the speed of light, allowing for the rotation and mass of the black hole.