Different users will have a term they use, and some may be more popular than others, or more specific to a certain situation. Blizzard, themselves, refrain from using a specific term for the behavior, when talking about it in a blog concerning graphical improvements in the upcoming expansion, Legion. Instead, they simply explain why it happens.
I will offer an explanation on both Possible Terms and Related Terms, and suggest that you choose your terminology for the situation. If you have to bring up the behavior in the middle of a raid, you do not want to add to the confusion by having to explain technical jargon to 30+ other players. Likewise, more technical terms can more accurately define the exact occurrence you are experiencing.
I will also explain Draw Distance, the mechanic behind the behavior you are experiencing, to help further explain both the terms and why this is actually happening.
OK, so you know why it happens, now. But what do you call it? My recommendation: whatever you want to. The thing about draw distance is that it exists in some form in basically every video game you have ever played. Assuming your computer meets the recommended system requirements for running World of Warcraft, you should never run into a situation where you were adversely effected by your ability to see further than the maximum draw distance. As a result, you should never run in to a situation where it is imperative that you inform other players of the limitations of your draw distance. It just doesn't happen.
- Being drawn: The term suggested by two bugs is quite accurate. At the end of the day, the objects are "being drawn" once they fall inside that draw distance. This is a very good general term to use, when describing the behavior to other players.
- Objects appearing: Another cover-all term to further emphasis my point. In a game-related nature, you will likely only be talking about the occurrences in context of being a video game player, not a computer graphics engineer. You have a much better chance of other users understanding you if you use non-technical terms. If you run into one of those players that insist on correcting you with correct terminology, hear them out. Report back, if you can. Ultimately, it is better to go for terms that are more likely to be understood by the audience, and it is a fair assumption that the average World of Warcraft player will not be versed in computer graphics.
- Popping: To say "the objects pop in and out" accurately describes exactly what is happening under particular context. As you have found through research, the term "pop" in computer graphics is commonly associated with a level of detail issue. However, "Popping" can also refer to the action of committing a set of graphics to be displayed, in the same way the draw distance determines that the objects should be committed to the display as they appear. Consider that this term would be considered jargon of the computer graphics industry. Unless the person you are speaking to understands computer graphics, the term may not be understood as you intend it. This is also the term used to describe the behavior on the Wikipedia page for Draw Distance, in detailing the issues that arose from shorter draw distances in earlier video games.
- Clipping: You could say that objects were "clipped", when they disappear from the screen. Clipping refers to the process where objects in front of the minimum draw distance, and objects further past the maximum draw distance, are removed from the display pre-render. You would not use this term to describe the opposite, where objects appear after moving inside of the draw distance.
- Artifacts: Depending on your perception of the behavior, and indeed, the actual reason it is behaving that way, you might consider the game to be displaying visual artifacts. Visual artifacts are generally undesired changes to graphical interfaces, often resulting in things looking 'off'. Some define artifacts as resulting from undesired but known issues with the graphics system, others define artifacts as resulting from specific malfunction with graphical hardware. Both could apply to World of Warcraft, given certain situation2. Again, this is a technical term, and has different meanings to different fields. It is best you do not use this term in general conversation, to avoid confusion.
There are some other terms you may come across in relation to draw distance, and the 'popping' effect of game objects. It might be worth addressing these terms, and the correct use, in understanding the terms listed above.
- LOD (Level of Detail): As the object moves further away, it may have its general quality or texture quality lowered to save on processing power. If done correctly, the change occurs at distances where you would not generally notice the change in quality. Level of Detail refers to the general quality of the object, as opposed to when it will appear or disappear. A good example of this can be seen in the comparison of two renders of Kalador, below.
What is it?
Changing the View Distance does not just mitigate the behavior - it directly relates to why it is behaving that way, in the first place.
When you play any game, you are typically looking through a "game camera". In Game Development these cameras can become quite complex, but in layman terms, they simulate real cameras to determine how to render the scene to the computer monitor1.
One important value a game camera holds is it's minimum and maximum draw distance. This tells the camera exactly what it can see, to draw to the screen. Before rendering any game object, the camera determines whether the object falls between the minimum and maximum draw distances. Depending on the circumstance, the camera may entirely ignore an object that crosses outside of the draw distance boundaries, or instead only draw the object as far (or as close) as the draw distance allows.
As you move forward, objects outside of the draw distance move closer. If these objects move inside of the draw distance, they will be drawn, but at this point, that is as simple as it gets. Ultimately, the object will "pop up" on the exact frame where the object is first within the draw distance. Likewise, it will disappear again, on the exact frame where the object first moves outside of the draw distance. By itself, this can result in some pretty rough looking graphics, so other things go into making it all look better.
- Increasing the "View Distance" from the graphics options: This is a common affordance, found in many other games. Ultimately, higher draw distances will result in more graphical processing, so lower-end graphic cards will not support longer draw distances as well as higher-end graphic cards.
- Developer-enforced "View Distance" limitations: Related to manually changing your "View Distance", the developers enforce a hard constraint on the limitations of your "View Distance". The maximum was raised from 777 to 1277 with the official Wrath of the Lich King release, and the bare minimum for the maximum was reduced to 0, with the release of Cataclysm. According to WoWWiki, the current working maximum view distance is approximately 185 - 1250. Anything below will not render the world correctly, and anything past will not have any noticeable effect. According to the previously mentioned Blizzard blog, further improvements have been made for Legion.
- Inclusion of other effects: Other graphical effects can be used to disguise the sudden appearance of far objects. World of Warcraft uses fog. At higher levels of detail, fog will cloud the space where objects 'appear', and make it far less obvious when they 'appear out of nowhere'.
1 OK, so it is not quite that simple, but for the purpose of this answer, that is as much as we need to know.
2 It would be desirable if we could draw "forever" (or as good as the human eye can see). In computing, this would lead to extensive resource use, and potential critical failure. As a result, the limitation enforced by the draw distance is a necessary, sometimes undesired, solution to a known graphical issue. Alternatively, efforts to smooth the draw distance could conflict with poor graphic card configurations, especially in cases where the graphic card is itself faulty.