An event horizon is the scientific jargon-name for the point at which you can no longer see stuff happening.
Scientists first isolated the event horizon in the 1950s, during Cold War attempts to create a superweapon to defeat the other side. In 1957 a naked event horizon was shown in New York to an audience of scientific journalists, who were suitably impressed by the discovery that sometimes you can't see things for some reason.
While no prizes were won for the discovery, the names of the scientists are still remembered and cited in many articles, such as this one.
Types of Event Horizon
Since the initial discovery, much work has been done on this fascinating topic, including categorisation of the different types of reasons why stuff becomes invisible.
Most commonly, an event horizon is stuff that is in the way of other stuff. Most people have encountered this form of horizon, as it has many manifestations, like the person who walks in front of the television at a crucial moment in the plot. Scientists have categorised these as Stuff Event Horizons, defining them as "a horizon composed of something that prevents you from seeing another thing that is obscured by that thing that is forming the Stuff Horizon".
In many cases, stuff horizons can be countered by moving slightly in one direction or another. This is not always possible in the case of larger stuff horizons (such as that which causes a solar eclipse) but is usually the way forward. Many stuff horizons are temporary.
A Farness Event Horizon is when stuff can no longer be seen because it is too far away from the observer. Farness horizons vary based on eyesight, atmosphere and lighting. For this reason, a farness horizon is very much dependent on individual circumstances.
On Earth, a farness horizon will eventually become a stuff horizon, as the curvature of the Earth means that there is planet in the way of itself. This is why it is not possible to see all the way to the other side of the planet, although the gradual uncurving of the Earth may eventually cause this sort of horizon to disappear.
Atypical Event Horizons are event horizons caused by something that has nothing to do with these more common horizons. For instance, an object that ate light could be considered an AEH despite the appearance that this would merely be an extreme example of a Farness Horizon. Similarly, if the stuff being observed were to become invisible, such an occurrence would be atypical indeed.
Event Horizons in Fiction
The film Event Horizon follows a group of submariners as they travel into the deeps of the oceans to observe stuff and find that they can no longer see anything. Then things begin to turn gory. As the film progresses, the philosophical and theological complexities of the varying kinds of event horizon are discussed in detail by Sam Neill.