D. Both in the English and Hebrew alphabets the fourth letter, whose numerical value is four. The symbolical signification in the Kabbala of the Daleth is “door”. It is the Greek delta D, through which the world (whose symbol is the tetrad or number four,) issued, producing the divine seven. The name of the Tetrad was Harmony with the Pythagoreans, “because it is a diatessaron in sesquitertia”. With the Kabbalists, the divine name associated with Daleth was Daghoul.
Source: H.P.Blavatsky - The Theosophical Glossary