Euclid ( ; Eukleides), fl. 300 BC, also known as Euclid of Alexandria, was a Greek mathematician, often referred to as the "Father of Geometry". He was active in Alexandria during the reign of Ptolemy I (323–283 BC). His Elements is one of the most influential works in the history of mathematics, serving as the main textbook for teaching mathematics (especially geometry) from the time of its publication until the late 19th or early 20th century. In the Elements, Euclid deduced the principles of what is now called Euclidean geometry from a small set of axioms. Euclid also wrote works on perspective, conic sections, spherical geometry, number theory and rigor.

Euklid von Alexandria (altgriechisch  ; latinisiert ; ca. 360 v. Chr. bis ca. 280 v. Chr.) war ein griechischer Mathematiker.

Euclide, en grec ancien Εὐκλείδης Eukleidês (né vers -325, mort vers -265 à Alexandrie) est un mathématicien de la Grèce antique, auteur des Éléments, qui sont considérés comme l'un des textes fondateurs des mathématiques modernes.

Euklides z Aleksandrii (gr. , Eukleides, ur. ok. 365 p.n.e., zm. ok. 300 p.n.e.) – matematyk grecki pochodzący z Aten, przez większość życia działający w Aleksandrii.