Euclid was an ancient Greek mathematician active in Alexandria during the reign of Ptolemy I. He is famously known as the 'father of geometry' for his foundational contributions to the field. His masterwork, 'Elements', is one of the most influential works in the history of mathematics, serving as the primary textbook for the subject for over two millennia. The principles laid out in his work, known as Euclidean geometry, remained the undisputed standard until the development of non-Euclidean geometry in the 19th century.