I'm with @DanThomas on this.
- Apple branded JavaScript for Automation as "JXA" when it was released, and have used it a number of times since.
- Formal Apple docs probably don't use it because, well, they are "formal" documents. Just like Peter doesn't like to use "KM" in the wiki.
- A google search turns up about the same hits for both, but "JavaScript for Automation" also pulls in other JavaScript, non-JXA stuff.
- Most of the early third-party sites and blogs use "JXA", like the dtinth/JXA-Cookbook Wiki
- In this forum, we have a tag for "jxa" and most often refer to it as "JXA".
When in doubt, use both. In fact, I have a text expansion just for ";jxa":
"JavaScript for Automation (JXA)"