In Kabbalah, Sefirot are a series of concealments between the creature and the Creator. They are known separately as Keter, Hochma, Bina, Hesed, Gevura, Tifferet, Netzah, Hod, Yesod, and Malchut. They are known collectively as Etz Chaim (Tree of Life). There is also another "invisible" Sefirot known as Da'at, which is technically the GE Galgalta Eynaim de ZON of the world of Nikudim.
The Sefirot exist within a number of worlds, and according to their current Behina (phase, see Four Phases of Direct Light) they are given different names. For example, in the world of AK Adam Kadmon, Keter is named Galgalta, Hochma is AB, Bina is SAG, and so on. These names specify the specific development of the creature as it perceives the Ohr.
In Hebrew, Sefira means "enumerations" but also "sparkling". Sefirot are used to enumerate the Ohr (Upper light) when they are affected by Ohr Makif, but when undertaking Ta'amim in the Toch they are said to "sparkle" with the Ohr Pnimi.