Tau, spelled out in full: Tau Vau

Tau is the equal-armed cross, attributed to Saturn/Earth and the World/Universe card. 

Vau is the nail, attributed to Taurus and the Hierophant card. 

Each represents the mysteries in completion and balance. Vau represents the mysteries withdrawn and secret, or reduced (contracted) to a systematic teaching, even a single word, name or symbol. Tau represents the mysteries expanded, unfolded, in motion, fully manifested. Together they are the point and the extension of that point into the cross. 

The Hierophant is the teacher, the initiator, the expounder of the mysteries, as well as the dispenser of hidden wisdom. The Universe is that same wisdom revealed and yet concealed within the vast expanse of the manifest. The Universe manifests according to the same cosmic laws which are codified and revealed by the Hierophant. 

The nail attaches spirit to matter, nailing the Rose to the Cross. Spirit is represented in the World/Universe card by the dancing woman, Anima Mundi or the world soul. She is a reflection of AIMA, the Great Mother of Binah, which is reflected in the attribution of the path of Tau to Saturn. 

In Crowley's deck, the Universe and the Hierophant are the only two cards with the Kerubs at the quarters. In the Universe card, the Kerubs are all shown facing outward; in the Hierophant card, the Eagle and the Man face inward. (James Eshelman gives the only plausible explanation I've ever heard for Crowley's placement of the Kerubs on these cards. It can be found in several places on the Temple of Thelema website: http://www.heruraha.net)

The Universe represents Nuit as infinite expansion, and the Hierophant Hadit as infinite contraction. These attributions are shown also by the forms of the letters themselves: Tau resembles the vault of the body of the heavens, and Vau the flaming torch at the center of the earth. 

"I am the blue-lidded daughter of Sunset; I am the naked brilliance of the voluptuous night-sky."

Liber AL, I: 64

"I am the flame that burns in every heart of man, and in the core of every star."

Liber AL, II: 6

The letters Tau and Vau add up to 406, which is also the enumeration of Atoh. Gikatalia's Gates of Light link the word Atoh (Aleph Tau Heh) to the Torah as follows: The first two letters are Aleph and Tau, the first and the last letters of the Hebrew alphabet, thus representing all 22 letters. The third letter in Atoh is Heh, which enumerates to 5. The Torah consists of 5 books written with 22 letters. 

The Zohar states that the Torah was created prior to the creation of the world, and was a blueprint for that creation. Torah is derived from the root Tau Resh Heh, which means 'to teach', though it is commonly accepted as meaning 'law'. Modern Qabalah (from Waite?) asserts that metathesis links TORA (law) with TARO and ROTA (wheel - interestingly, in Waite's deck, the two cards that show the Kerubs are the World and the Wheel of Fortune), as well as ORAT (to speak) and ATOR (vaguely linked to Hathoor, who stands in the East, behind the Hierophant, in the Golden Dawn Neophyte Initiation).