Ain Soph (from Hebrew "no end"): according to the Kabbalah, the Infinite Being that is inaccessible to any human knowledge.
"Kabbalah" means "tradition" and refers to a school of Jewish mysticism that originated in the Middle Ages in the French region of Provence and was further developed in Spain and Palestine. According to the Kabbalah, the 32 ways of God's wisdom can be derived from the 22 consonants of the Hebrew alphabet together with the 10 original numbers, also known as the Sephiroth. In the cabbalistic diagram, the sephiroth correspond to ten circles that are interconnected by 22 lines. This results in seven vertical levels or spheres that can be traversed in various ways.
With regard to its infinite character, the concept of Ain Soph far exceeds the capacity of human understanding and hence, of course, the descriptive resources available to the author of this dictionary. Nonetheless, we shall continue with our attempt at describing it. Ain Soph is really God, who — as far as we know — is not directly accessible to human understanding either, but can be experienced only via His manifestations and impacts on this world. Likewise, the Ain Soph can be experienced only via Its manifestation in the form of the ten sephiroth. In Its true form, it is like a ray of light of unlimited brightness stretching out toward infinity. Wherever the light meets space, the latter contracts and generates the circles of the sephiroth. Thus, the sephiroth result from the interaction between the Infinite Being and finite space.
In the diagram you can see the seven spheres or hierarchies vertically arranged one above the other. We can picture the Ain Soph to be located above the diagram. Each of the sephiroth is assigned a number and a human quality. The top circle, which is closest to the Ain Soph, represents the "crown"; beneath it we find "wisdom", "intelligence", "mercy", and so forth, until we reach the bottom circle, which represents earthly "kingdom". Only by traversing all the sephiroth from the bottom to the top could we come close to the Ain Soph, though even then we could never quite reach it.
If you think you have not fully understood the above description, don't worry! You should be at least 40 years old to study Kabbalah. And even then you can only hope to come somewhat close to comprehension, but can never quite reach it.