What is Ein Sof?

"The term Ein Sof is sometimes taken in Kabbalah to refer to God’s very essence"
https://www.inner.org/worlds/einsof.htm

"Kabbalists believed that God is impossible to comprehend. That there is a gap between how mankind can understand God and what God actually is. Kabbalists call this unknowable aspect of God Ein-Sof"
https://www.deliriumsrealm.com/kabbalah-ein-sof-sefirot/

"EIN-SOF (Heb. אֵין סוֹף; "The Infinite," lit. that which is boundless), name given in Kabbalah to God transcendent, in His pure essence: God in Himself, apart from His relationship to the created world."
https://www.jewishvirtuallibrary.org/ein-sof

"Ein-Sof, the Infinite God, has no static, definable form. Instead, the Kabbalists conceive God, the world and humanity as evolving together through, and thus embodying, a number of distinct stages and aspects, with later stages opposing, but at the same time encompassing, earlier ones."
http://www.newkabbalah.com/einsof.html