Adelard of Bath (Adelardus Bathensis; 1080? - 1142–1152?) was a 12th-century English natural philosopher. He is known both for his original works and for translating many important Greek scientific works of astrology, astronomy, philosophy, alchemy and mathematics into Latin from Arabic versions, which were then introduced to Western Europe. The oldest surviving Latin translation of Euclid's Elements is a 12th-century translation by Adelard from an Arabic version. He is known as one of the first to introduce the Arabic numeral system to Europe. He stands at the convergence of three intellectual schools: the traditional learning of French schools, the Greek culture of Southern Italy, and the Arabic science of the East.
Adelard's biography is incomplete in places, and leaves some aspects open to interpretation. Consequently much of what is ascribed to Adelard is a product of his own testimony.
Adelard claims to come from the Roman English city of Bath. How he lived is not entirely known. Despite his extensive travels, by the end of his life he is thought to have returned to Bath, where he died.
The parents of the philosopher are not known for sure, but Fastred, a tenant of the Bishop of Wells, is noted by scholars as a possible father. The name Adelard is of Anglo-Saxon origin, which would make him to be of low status in 11th-century England. It is believed that he left England toward the end of the 11th century for Tours, likely on the advice of Bishop John de Villula, who had moved the seat of his bishopric from Wells to Bath in 1090. During his studies in Tours, an anonymous "wise man of Tours" inspired Adelard with his interest in astronomy to study the science. Adelard later taught for a time at Laon, leaving no later than 1109 for travel.
After leaving Laon, Adelard describes himself as travelling to Southern Italy and Sicily no later than 1116. Adelard also reports extensive travel throughout the "lands of the Crusades": Greece, West Asia, Sicily, possibly Spain, Tarsus, Antioch, and potentially Palestine.