Neidan, or internal alchemy (traditional Chinese: 內丹術; simplified Chinese: 內丹术; pinyin: nèidān shù), is an array of esoteric doctrines and physical, mental, and spiritual practices that Taoist initiates use in hopes to prolong life and create an immortal spiritual body that would survive after death. Also known as Jindan (金丹 "golden elixir"), internal alchemy combines theories derived from external alchemy (waidan 外丹), correlative cosmology (including the Five Phases), the emblems of the Yijing, and medical theory, with techniques of Taoist meditation, daoyin gymnastics, and sexual hygiene.

In neidan practice, Taoists claim the human body becomes a cauldron (or "ding") in which the Three Treasures of Jing ("Essence"), Qi ("Breath") and Shen ("Spirit") are 'cultivated' for the purpose of improving physical, emotional and mental health, and ultimately returning to the primordial unity of the Tao, i.e., attaining Taoist Immortality. It is believed the Xiuzhen Tu is such a cultivation map. In China, this act is an important form of practice for most schools of Taoism.

The Chinese compound nèidān combines the common word nèi 內 meaning "inside; inner; internal" with dān 丹 "cinnabar; vermillion; elixir; alchemy". The antonym of nèi is wài 外 "outside; exterior; external", and nèidān "internal elixir / alchemy" was coined from the earlier complementary term wàidān 外丹 "external elixir / alchemy".

Traditional Chinese medical alchemical texts and sources ordinarily call neidan the jīndān dào 金丹道 or Way of the Golden Elixir. In Modern Standard Chinese usage, the term nèidān shù 內丹術 (with 術 "art; skill; technique; method") refers generally to internal alchemical practices for longevity by maintaining good health, and the prevention of pain and suffering.

The date for the earliest use of the term neidan is uncertain. 內丹 or neidan had been mentioned in 灵剑子 by Xu Xun 许逊 in Jin dynasty (266–420), but on the other hand Arthur Waley proposed that it was first recorded in the 559 vow taken by Tiantai Buddhist patriarch Nanyue Huisi praying to successfully make an elixir that would keep him alive until the coming of Maitreya. Many scholars agreed, including Joseph Needham and Lu Gwei-djen who translated Huisi's vow to live as an ascetic in the mountains:

I am seeking for the longevity in order to defend the Faith, not in order to enjoy worldly happiness. I pray that all the saints and sages will come to my help, so that I may get some good magic mushrooms [zhi 芝], and numinous elixirs [shendan 神丹], enabling me to cure all illnesses and to stop both hunger and thirst. In this way I shall be able to practice continually the way of the Sutras and to engage in the several forms of meditations. I shall hope to find a peaceful dwelling in the depths of the mountains, with enough numinous elixirs and medicine to carry out my plans. Thus, by the aids of external elixirs [waidan] I shall be able to cultivate the elixir within [neidan].

Others believed that neidan first occurred in the biographies of Deng Yuzhi 鄧郁之 (fl. 483–493) and Su Yuanming 蘇元明 (fl. c. 600). However, the authenticity of the relevant passages mentioned above is doubtful.

The term neidan was seldom used throughout the late Tang dynasty (618–907) and Five dynasties (907–960) period, and only became widespread around the beginning of the Song dynasty (960–1279) period, when neidan evolved into a highly complex system in both its theoretical and practical aspects. Tang texts described internal alchemical practices with the words fúyào 服藥 "take drug/medicine" and chángshēng 長生 "long life, longevity; (Taoism) eternal life". Liu Xiyue's 劉希岳 988 Taixuan langranzi jindao shi 太玄朗然子進道詩 (Master Taixuan Langran's Poems on Advancing in the Tao) has the earliest datable mention of the terms neidan and waidan. The c. 1019 Yunji Qiqian Taoist anthology mentions the term neidan.