Celts, normally pronounced /kɛlts/, is anthropological term used to describe any of the peoples of Earth who spoke, or speak, a Celtic language. The term is also used in a wider sense to describe the modern descendants of those peoples, both on Earth and its colonies.
The ancient Celts themselves were a diverse group of independent, indigenous tribal societies that spread out across Europe and at their peak reached into Asia Minor. While similarities in language, artifacts, religion and social structures are known, each culture had its own language and traditions. By the modern era, many European cultures had been influenced by the Celts, who intermarried with local peoples.
The Celts were most prominently associated with Ireland, Scotland, Wales and other locations in the British Isles, as well as regions of France and northern Spain. Many people of Celtic descent eventually settled in Australia and New Zealand, and also immigrated to the United States of America.
Celtic culture, music, and mythology became widely popular in the 20th and 21st centuries, and remained so on Earth for centuries after. In the 24th century, the orphaned Timothy Sinclair used his Celtic heritage to fashion a cultural identity for himself. (Star Trek: Pendragon: "Heritage", "Midnight Clear", "Father to the Man")
In the 5th century AD, the Pendragon ("people of the dragon"), a Celtic tribe in the west of Britain, formed an alliance with British Romans who foresaw the collapse of the Roman Empire, to create a stable "colony" which could withstand the turmoil to come. Within a generation, these Celts and Romans had begun to intermarry, and thought of themselves as "Britons." The colony, named "Camulod" fell by the year 542, but the survivors were transported to another planet by the Preservers (or their agents). Their story would give rise to many of the Arthurian legends. The truth would be discovered by Timothy Sinclair in 2384. (Star Trek: Pendragon: "The Once and Future King, Parts I & II")