Celadon (/ˈsɛlədɒn/) is a term for pottery denoting both wares glazed in the jade green celadon color, also known as greenware or "green ware" (the term specialists now tend to use), and a type of transparent glaze, often with small cracks, that was first used on greenware, but later used on other porcelains. Celadon originated in China, though the term is purely European, and notable kilns such as the Longquan kiln in Zhejiang province are renowned for their celadon glazes. Celadon production later spread to other parts of East Asia, such as Japan and Korea, as well as Southeast Asian countries, such as Thailand. Eventually, European potteries produced some pieces, but it was never a major element there. Finer pieces are in porcelain, but both the color and the glaze can be produced in stoneware and earthenware. Most of the earlier Longquan celadon is on the border of stoneware and porcelain, meeting the Chinese but not the European definitions of porcelain.

For many centuries, celadon wares were highly regarded by the Chinese imperial court, before being replaced in fashion by painted wares, especially the new blue and white porcelain under the Yuan dynasty. The similarity of the color to jade, traditionally the most highly valued material in China, was a large part of its attraction. Celadon continued to be produced in China at a lower level, often with a conscious sense of reviving older styles. In Korea, the celadon produced during Goryeo period (918–1392) are regarded as classic wares of Korean porcelain.

The celadon color is classically produced by firing a glaze containing a little iron oxide at a high temperature in a reducing kiln. The materials must be refined, as other chemicals can alter the color completely. Too little iron oxide causes a blue color (sometimes a desired effect), and too much gives olive and finally black; the right amount is between 0.75% and 2.5%. The presence of other chemicals may have effects; titanium dioxide gives a yellowish tinge.

The term "celadon" for the pottery's pale jade-green glaze was coined by European connoisseurs of the wares. The most commonly accepted theory is that the term first appeared in France in the 17th century and that it is named after the shepherd Celadon in Honoré d'Urfé's French pastoral romance L'Astrée (1627), who wore pale green ribbons. (D'Urfé, in turn, borrowed his character from Ovid's Metamorphoses V.144.) Another theory is that the term is a corruption of the name of Saladin (Salah ad-Din), the Ayyubid Sultan, who in 1171 sent forty pieces of the ceramic to Nur ad-Din Zengi, Sultan of Syria.

Celadon glaze refers to a family of usually partly transparent but colored glazes, many with pronounced (and sometimes accentuated) "crackle", or tiny cracks in the glaze produced in a wide variety of colors, generally used on stoneware or porcelain pottery bodies.

So-called "true celadon", which requires a minimum 1,260 °C (2,300 °F) furnace temperature, a preferred range of 1,285 to 1,305 °C (2,340 to 2,380 °F), and firing in a reducing atmosphere, originated at the beginning of the Northern Song dynasty (960–1127), at least on one strict definition. The unique grey or green celadon glaze is a result of iron oxide's transformation from ferric to ferrous iron (Fe₂O₃ → FeO) during the firing process. Individual pieces in a single firing can have significantly different colors, from small variations in conditions in different parts of the kiln. Most of the time, green was the desired color, reminding the Chinese of jade, always the most valued material in Chinese culture.

Celadon glazes can be produced in a variety of colors, including white, grey, blue and yellow, depending on several factors:

The most famous and desired shades range from a very pale green to deep intense green, often meaning to mimic the green shades of jade. The main color effect is produced by iron oxide in the glaze recipe or clay body. Celadons are almost exclusively fired in a reducing atmosphere kiln as the chemical changes in the iron oxide which accompany depriving it of free oxygen are what produce the desired colors.