Circassia (/sɜːrˈkæʃə/ sir-KASH-ə), also known as Zichia, was a country and a historical region in Eastern Europe. It spanned the western coastal portions of the North Caucasus, along the northeastern shore of the Black Sea. Circassia was conquered by the Russian Empire during the Russo-Circassian War (1763–1864), after which approximately 80–97% of the Circassian people were either exiled or massacred in the Circassian genocide.

In the medieval era, Circassia was nominally ruled by an elected Grand Prince, but individual principalities and tribes were autonomous. In the 18th–19th centuries, a central government began to form. The Circassians also dominated the northern end of the Kuban River, but were eventually pushed back to the south of the Kuban after suffering losses to military raids conducted by the Mongol Empire, the Golden Horde, and the Crimean Khanate. Their reduced borders then stretched from the Taman Peninsula to North Ossetia. The term Circassia is also used as the collective name of various Circassian states that were established within historical Circassian territory, such as Zichia.

Legally and internationally, the Treaty of Belgrade, which was signed between Austria and the Ottoman Empire in 1739, provided for the recognition of the independence of Eastern Circassia. Both the Russian Empire and the Ottoman Empire recognized it under witness from the other great powers of the time. The Congress of Vienna also stipulated the recognition of the independence of Circassia. In 1837, Circassian leaders sent letters to a number of European states requesting diplomatic recognition. Following this, the United Kingdom recognized Circassia. However, following the outbreak of the Russo-Circassian War, the Russian Empire did not recognize Circassia as an independent nation and instead treated it as Russian land under rebel occupation, despite having no control or ownership over the region. Russian generals often referred to the Circassians as "mountaineers", "bandits", and "mountain scum" rather than by their ethnonym.

The Russian conquest of Circassia created the Circassian diaspora; the overwhelming majority of Circassians today live outside of their ancestral homeland, mostly in Turkey and other parts of the Middle East. Only about 14% of the global Circassian population lives in the modern-day Russian Federation.

The words Circassia and Circassian (/sərˈkæsiənz/ sər-KASS-ee-ənz) are exonyms, Latinized from the word Cherkess, which is of debated origin. One view is that its root stems from Turkic languages, and that the term means "head choppers" or "warrior killers" accounting for the successful battle practices of the Circassians. There are those who argue that the term comes from Mongolian Jerkes, meaning "one who blocks a path". Some believe it comes from the ancient Greek name of the region, Siraces. According to another view, its origin is Persian.

In languages spoken geographically close to the Caucasus, the native people originally had other names for the Circassia, but with Russian influence, the name has been settled as Cherkessia/Circassia. It is the same or similar in many world languages that cite these languages.

Circassians themselves don't use the term "Circassia", and refer to their country as Адыгэ Хэку (Adıgə Xəku) or Адыгей (Adıgey).

Another historical name for the country was Zichia (Zyx or the Zygii), who were described by the ancient Greek intellectual Strabo as a nation to the north of Colchis.