Shin, spelled out in full: Shin Yod Nun

Shin is the tooth, attributed to Fire/Spirit and Judgment or The Aeon.

Yod is the hand, attributed to Virgo and The Hermit.
Nun is the fish, attributed to Scorpio and Death.

The path of Shin leads from Malkuth (represented by Virgo, an Earth sign) to Hod (represented by Scorpio, a Water sign).

Yod is primal fire (as Y of YHVH), the seed, potential and unmanifest, invisible and eternal, outside of time and space, it is the uppermost point of the tetrahedron.
Shin is fire manifested, the unfolding of the seed in three-fold form (active, passive and equilibrated), it represents the sides of the tetrahedron (in which the base is implied).
Nun represents the transformation from one state of fire to the other. The spiritual seed dies to eternity and is reborn into the temporal world (the path of involution). The life manifested in time and space is transcended in higher consciousness, reborn in spirit.
Nun is also the fish, which is the part of the Self that is not dissolved in the Sea of Binah when the outer self is transcended (or no longer identified with). It is the Neschamah or eternal consciousness, the immortal part of the soul. (The waters of Binah are reflected down into the watery Sephira Hod).

One of the motifs of the path of Shin is resurrection. This is supported by the Hermit as Anubis, the guide through the underworld, and by Death in its 3 stages symbolized by the scorpion, the snake and the eagle.

Yod, Nun and Shin (in that order) equate to IAO as the formula of Isis Apophis and Osiris.
Yod is the Isis phase, in which the seed is planted. This corresponds to the initial effort in any task, the beginning of the learning process in which is present the seed of mastery.
Nun is the Apophis phase, in which the seed gestates in the darkness, dying to its previous form so that a new form can arise. In the learning process, this corresponds to the phase in which the initial interest and inspiration dims, in time fading to nothing. The task of learning becomes dull and repetitive, whatever fruits may lie ahead cannot be seen.
Shin is the Osiris phase, in which the new plant sprouts and flowers. Mastery is attained and a new cycle of learning is begun.
Resurrection of life is also found in the seed after the death of the fruit.

The letters Shin Yod Nun, with their attributions of fire, Virgo and Scorpio, also bring to mind the Latin phrase 'Igne Natura Renovatur Integra' (I.N.R.I.), meaning 'nature is constantly renewed by fire' (or 'matter is restored by spirit'). As an alchemical formula, this phrase corresponds exactly with the processes undergone on the path of Shin, in which the subtle fire brings about a transformation to the dross, or outer nature, during which it is redeemed in spirit.

The fire of Shin prepares the vehicle for the reception of higher consciousness, the beginning of the awareness of the Neschamah in the form of higher intuition. Hod (the destination of the path of Shin) is a watery Sephira, symbolized by the cup and having the nature of revelation. Part of the Practicus grade is the illumination of the outer forms of the archetypes, the beginning of true spiritual perception (Understanding, or knowledge of the soul). These forms are illuminated by the lamp of occult knowledge shown in the Hermit card (ultimately derived from the primal fire of Yod).

In the Aeon of Osiris, the aspirant suppressed the passions, starving them out of the psyche so that they might die completely to the outer world and be reborn in God. In the Aeon of Horus, the aspirant learns to use the passions in order to exalt the consciousness, turning them towards the manifestation of the Chiah to which they have subjugated the outer persona. Both of these process involve a very real death experience (several, in fact) and both of them are aptly symbolized by the letters Shin Yod Nun.