The Concise Oxford Dictionary of World Religions (2421 page)

1. Chin., ‘cinnabar’, the most important element in religious
Taoism
, in the pursuit of immortality. In Outer
Alchemy
,
wai-tan
, effort was devoted to purifying tan and converting it to gold, to prolong life. In Inner Alchemy, tan is understood to be the interactive energy of
yin-yang
produced by appropriate breathing.
2. The slip of paper which designates the sitting-place of a Zen monk for
zazen
.
3. Earth-bound component of the body in Zoroastrianism: see
FRAVASI
.