Kether is the first Sephiroth, at the top of the Tree of Life, and is usually taken to be identified with the idea of the most primordial and pure energy possible, the first thing created.
When I thought of writing something here, I was under the impression that there were already writeups inside of this node. There may have been at one point, but it is gone now: although much of what was written here was probably what you could find in any New Age Encyclopedia. After 2000 years, the basic structure of the Tree of Life has been sketched out very thoroughly by a variety of commentators. Of course, the "primary material" for Cabala (or whatever regular expression you want to use for that) never lays out a nice and neat diagram of the universe anymore than the gospels have the schedule for next Sunday's worship at the local Presbyterian meeting. These things are abstracted out of a some very complicated and esoteric material. The Zohar calls Kether "the concealed of the concealed", which is a bit more mysterious than simply picturing it as the battery in a wiring diagram.
The meaning of the word is one hint, or lack thereof into what Kether is. The name means "crown", from a root word meaning "circular". There are actually several other words in Hebrew for "crown", and Kether is a very obscure choice, since it is a Hapax Legomen, occurring only in one passage in the bible, in the Book of Esther. Although any caballistic term sounds mysterious to an English speaker, most of them were actually prosaic terms that would have formed part of common Hebrew. The next two sephiroth, Chokmah and Binah, are just the terms for "wisdom" and "intelligence", and presumably wouldn't have sounded esoteric to Hebrew-speaking people. And yet Kether is an odd word, with an odd meaning. I have read many possible interpretations of it. The most obvious one is that crowns bring to mind power and majesty. And that it represents something that is above and beyond the brain and the mind. From the root word of "circular", it could be guessed that it represents the idea of circularity and perfection. Of course, any time you have a word with ambiguous meanings, the possible metaphorical meanings of it quickly multiply. Especially since depending on which interpretation you are reading, Kether is also identified with Da'ath (Knowledge), Ratzon (Will) and Ayin (non-existence).
Another question to ask is where exactly Kether fits in cross-culturally. Is it the same as Sunyatta in Buddhism? As al-Aql for the Druze? As the mysterious 4th Plateau or Omega Point or Instrumentality? I would say that since its intracultural meaning is by definition, almost impossible to guess, its extracultural meaning is nothing that could be described conveniently.
But, of course, what does one expect from the ineffable?