Purely and simply, a glitch in a video game is when something happens that was not initially intended to happen by the video game programmer. In the early days of video games, when there were fewer lines of code, there were almost no glitches because the errors in the code were much more obvious and did not cause as many problems. However, now that there are multi-gigabyte games with several teams working on a single game, glitches are practically an expectation.

Glitches are much more than just errors that a compiler would catch. They are logical errors. Oftentimes, they result in a faulty collision detection or an odd graphic to appear simply because the programmer forgot to prevent it from happening and since the code is so long it is difficult to go through the entire thing and find where the problem is.

There is something of an ambiguity with glitches. Was the game intended to work that way or was it just a mistake that did not deviate so far from the general rules of the game as to be an obvious glitch? The best way to tell is to ask the creator, but for the creator to admit that there in fact are things in the game that were not intended to happen would require him to have an excessive amount of humility, which is hard to come by these days.