"When I was in the second grade, there was this other kid who always used to make fun of me. His name was Stupen Dumbello. I think he was from a foreign country. He was real fat and wore real thick glasses and had a bad complexion. One time after he made fun of me I said to him: "Someday, somebody's going to make fun of you." But I don't think they ever did."—Jack Handey
Tropes have exceptions. Sometimes, a writer uses a situation that seems indelibly married to a trope, but the trope never appears. It's not a Subverted Trope, because the lack of trope is never pointed out. Nor is it a Lampshade Hanging, because the presence of trope is never pointed out. The trope just isn't there.
That is the core meaning of "averted". The writer just went past the trope. He ignored it, he forgot it existed, or it wasn't even a trope at all in his culture.
Example of the difference between subverting and averting a trope:
Take the trope Incredibly Obvious Bug. The trope is that listening and tracking devices are huge and obvious with blinking lights, instead of being small, stealthy, and hidden.
- Played straight: A villain plants an Incredibly Obvious Bug on a chair. Despite the green flashing light and periodic beep, The Hero does not notice it.
- Subversion: A villain plants an Incredibly Obvious Bug on a chair. The Hero finds it and throws it out, muttering, "Does he think I'm blind?"
- Averted: A villain plants a bug on a chair. It is small, camouflaged and unobtrusive, and goes undetected by the hero. The trope never comes into play.
Even though There Is No Such Thing as Notability, averting is generally not an example for mentioning on a trope page, except for tropes that are so common that the list of aversions is actually shorter, such as Limited Wardrobe. The reason is that different people have different expectations. For example, say there is an action movie that does not have a single instance of Stuff Blowing Up. Just because you expect it does not mean that it was subverted or used in any manner. If it was not used, then it is not an example.
On the other hand, Space Is Noisy is so incredibly common in science fiction (especially TV and Film) that a work that manages to avoid it is worth mentioning. We don't want to have to scroll through examples like:
- Averted in Harry Potter, where nothing like this ever happens.
So aversions worth mentioning will generally follow this pattern: A supermajority of works that have element A will also have trope A, but work W has element A without trope A. If the number of aversions on a tropes page consist of at least a third of the examples, perhaps averting the trope isn't as notable as initially thought.
See Meta Trope Intro for a comparison with the ways that a trope can actually be used.