A term borrowed from India's ancient language, Sanskrit or devanagari ("divine language"). It can be translated by several English concepts ("origin", "source", "womb", "female genitals") and is, to my knowledge, the most respectful word available for naming what our modern languages refer to as vulva, pussy or cunt; or other equivalents.
The term yoni heralds from a culture and religion in which women have long been regarded and honored as the embodiment of divine female energy - the goddess known as Shakti - and where the female genitals are seen as a sacred symbol of the Great Goddess.
Because Tantrics and others worship the Divine in the form of a Goddess, the term yoni has also acquired other, more cosmic meanings, becoming a symbol of the Universal Womb, the Matrix of Generation and Source of All.
A page from the book: