Shin

Letter in Hebrew. Also means 300.

Described as a mother letter in the Sepher Yetzirah, one of three.

He caused Shin to reign in Fire and crowned it, and he combined one with the other, and sealed them, as heaven in the universe, as heat in the year, and as the head of Man and Woman. (from the Sepher Yetzirah)

Tarot correspondences
In the Golden Dawn system, shin is attributed to the tarot card Judgement, and path number 31 of the Tree of Life. This is the path from Hod to Malkuth.

In the older system by Eliphas Levi, it is attributed to The Fool.