Now that a few more Numenera questions are coming in, there's been some misunderstanding about whether or not there is an accent over the last 'e', which stems from the logo "NUMENÉRA".
There is no accent over the final 'e' in Numenera. The mark on the logo is styling, not an accent.
What do Official Sources Say?
I couldn't find a single instance of a diacritic on the site, including the "What in a Name" page, which is literally all about the name of the game.
There's no accent in the book text itself, even when it's writing the logo it out in smallcaps with a trademark sign.
My conclusion is that it doesn't actually have a diacritic.
Oddly enough, the stylistic accent on the e seems to be inaccurate in terms of the word’s origin.
As @AdrianoVaroliPiazza points out in a comment,
It's a Latin-rooted word. From en.wikipedia.org/wiki/Numen : Numen, pl. numina, ("an influence perceptible by mind but not by senses,is a Latin term for a potential, guiding the course of events in a particular place or in the whole world, used in Roman philosophical and religious thought. It's just that, in Spanish, we do use accents, and for me, "Numenéra" looked better, even though it'd actually be incorrect Spanish: accents are not written on the penultimate syllable of a word when the word ends in a vowel. But it still looked nicer :)
He’s mostly correct, but numenera is not a Latin word. The word numen is, but it’s also a Spanish word (with the same forms, s. numen pr. "numEn", and pl. numina pr. "numIna"), and Latin does not have the -era suffix while Spanish does: it implies the place or object where the suffixed noun is usually found. This suffix is explicitly -era not -éra, so numenéra would be incorrect as Adriano points out. The word would still be pronounced "numenEra" in Spanish, though.
That said, I think the correct form would be numinera rather than numenera, but either way numenéra is right out.
For those curious, by my estimation, the meaning of the word would probably be something close to a shrine: it’s a place where one finds a guiding spirit or perhaps an object containing a guiding spirit, or possibly the state of being a such a spirit (“spirit-ness”).