"Luminosity" is best glossed as "self-awareness", not "metacognition"

Since metacognition is thinking about your thoughts, and self-awareness means introspecting, which means thinking about your thoughts (or self-awareness can mean being aware that you have a separate personality, a self, which isn't how you use it), I would say these could be synonymous. To test whether my perception here is wrong, I went to wikipedia, Google, and the dictionary to see how they used those words. Here's what I discovered:

Both the internet and you are using self-awareness in a broader way to encompass emotions and states of mind. Metacognition is more narrow and is something I personally use more frequently when I'm referring to re-engineering my thought processes. I think the reason I initially selected "meta cognition" as a term to suggest is because the sequences are, to me, an invitation to think about thinking and re-engineer one's thinking processes, so I was interpreting your luminosity concept within that context. Also, the way that I introspect about my feelings and experiences, I'm pretty focused on getting down to the thoughts behind everything and re-engineering them. The way I experience self-awareness, self-awareness and metacognition are inseparable and may as well be synonymous, but I acknowledge that other people may do it differently.

If your focus is more on the emotional / state of mind type aspects (I have not read all of your luminosity articles to be able to see the patterns in how you use it) self-awareness is probably a closer synonym for luminosity.

I also discovered that you have one of the top five Google results for luminosity. Good job. However, I think you're more likely to show up when people are looking for light bulbs or similar than when they are looking for self-improvement materials, so in my view it was not the most optimal term to SEO.

Eliezer didn't make it up (and I didn't make it up from scratch

I took the word "luminosity" from "Knowledge and its Limits" by Timothy Williamson, although I'm using it in a different sense than he did.

I see that, but I did not say that Eliezer came up with that specific example.

so bringing it up under the grandparent is peculiar

Because luminosity is a term used in the sequences, it's relevant. Because it's an example of a word that's synonymous with a much more common word:

Luminosity, as I'll use the term, is self-awareness.

It is definitely relevant as an example of uneccessary jargon in the sequences.

It would not support the point that Eliezer is coming up with unneccessary jargon, but that's not my point, my real point is that the sequences have unneccessary jargon. That is what's important here.

Since the sequences would get more hits from search engines by using existing terms, LWers and other flavors of rationalists would be able to communicate more easily, and people would have to remember less terms overall if existing terms were used, do you think it would be of greater benefit to use "self-awareness" instead?

Note: For SEO reasons it may be safer to add the term self-awareness to the title rather than replace luminosity. (When you change your title entirely, it may look like search engine spamming and get you spam penalties. I will check whether this specific technique worked if you're interested.)