The Pill of Immortality, also known as xiandan (仙丹), jindan (金丹) or dan (丹) in general, was an elixir or pill sought by Chinese alchemists to confer physical or spiritual immortality. It is typically represented as a spherical pill of dark color and uniform texture, made of refined medical material. Colloquially and in Chinese medicine, the term can also refer to medicine of great efficacy.
The search for the pill was started several centuries BC ago and continued until 500 AD and was often based on noble metals such as mercury and gold. Its search was supported by the emperors and the nobility of China, with a strong tradition in Taoism. During the Qin dynasty, the founding Emperor Qin Shi Huang consulted sages and alchemists to seek such a pill to achieve eternal life.
The alchemical tradition in China was divided into two differing schools in the search for the pill of immortality. Taoist sects which advocated the attainment of immortality by consuming substances were very popular during the Eastern Han dynasty in the 2nd century AD and they were collectively known as the school of the "external pill", or Waidan (外丹). By contrast, "internal alchemy", or Neidan (内丹), was thought to create an immortal body within the corporeal body, and a variety of actions involving dietary, respiratory, and sexual practices and/or mental practices such as meditation were believed to cause immortality.