History of geometry
Classic geometry was focused in compass and straightedge constructions. Geometry was revolutionized by Euclid, who introduced mathematical rigor and the axiomatic method still in use today. His book, "The Elements" is widely considered the most influential textbook of all time, and was known to all educated people in the West until the middle of the 20th century.
Democritus
Democritus was also a pioneer of mathematics and geometry in particular. We only know this through citations of his works (titled "On Numbers", "On Geometrics", "On Tangencies", "On Mapping", and "On Irrationals") in other writings, since most of Democritus's body of work did not survive the Middle Ages. Democritus was among the first to observe that a cone and pyramid with the same base and height has one-third the volume of a cylinder or prism respectively .