Euclid was a Greek mathematician who lived in the 3rd century BCE. He is often referred to as the 'Father of Geometry' and is considered one of the most influential mathematicians of all time. He is best known for his work on geometry, which includes the Elements, a 13-volume work that is still used today as the basis for teaching geometry. Euclid's work laid the foundations for modern geometry and has been studied by generations of mathematicians.