The Kabalistic term for the Deity prior to His self-manifestation in the production of the world, probably derived from Ibn Gabirol's term, "The Endless One" (she-en lo tiklah).
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"Before He gave any shape to the world, before He produced any form, He was alone, without form and without resemblance to anything else.(part ii., section "Bo," 42b). In other words, "En Sof" signifies "the nameless being." Jewish Encyclopedia
Who then can comprehend how He was before the Creation?
Hence it is forbidden to lend Him any form or similitude, or even to call Him by His sacred name, or to indicate Him by a single letter or a single point. . . .
But after He created the form of the Heavenly Man [ ], He used him as a chariot  wherein to descend, and He wishes to be called after His form, which is the sacred name 'Yhwh'"
The three letters composing the word "En" () indicate the first three purely spiritual Sefirot.
"Any name of God which is found in the Bible can not be applied to the Deity prior to His self-manifestation in the Creation, because the letters of those names were produced only after the emanation."Images @ Eminpee Fotography