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Euclid's Elements (sometimes: The Elements, Greek: Στοιχεῖα Stoicheia) is a large set of math books about geometry, written by the ancient Greek mathematician known as Euclid (c.325 BC–265 BC) in Alexandria (Egypt) circa 300 BC. The set has 13 volumes, or sections, and has been printed often as 13 physical books (numbered I-XIII), rather than one large book. It has been translated into Latin, with the title "Euclidis Elementorum". It is the most famous mathmetical text from ancient times.
Euclid collected together all that was known of geometry in his time. His Elements is the main source of ancient geometry. Textbooks based on Euclid have been used up to the present day. In the book, he starts out from a small set of axioms (that is, a group of things that everyone thinks are true). Euclid then shows the properties of geometric objects and of whole numbers, based on those axioms.
The Elements also includes works on perspective, conic sections, spherical geometry, and possibly quadric surfaces. Apart from geometry, the work also includes number theory. Euclid came up with the idea of greatest common divisors. They were in his Elements. The greatest common divisor of two numbers is the greatest number that can divide evenly into both of the two numbers.
The geometrical system described in the Elements was long known simply as "geometry" and was considered to be the only geometry possible. Today, that system is referred to as Euclidean geometry, to distinguish it from other so-called non-Euclidean geometries which mathematicians discovered in the 19th century.