几何学简称几何。几何学是数学的一个基础分支，主要研究形状、大小、图形的相对位置等空间区域关系以及空间形式的度量。

许多文化中都有几何学的发展，包括许多有关长度、面积及体积的知识，在西元前六世纪泰勒斯的时代，西方世界开始将几何学视为数学的一部份。西元前三世纪，几何学中加入欧几里德的公理，产生的欧几里得几何是往后几个世纪的几何学标准。阿基米德发展了计算面积及体积的方法，许多都用到积分的概念。天文学中有关恒星和行星在天球上的相对位置，以及其相对运动的关系，都是后续一千五百年中探讨的主题。几何和天文都列在西方博雅教育中的四术中，是中古世纪西方大学教授的内容之一。

Geometry (from the Ancient Greek: γεωμετρία; geo- "earth", -metron "measurement") is a branch of mathematics concerned with questions of shape, size, relative position of figures, and the properties of space. A mathematician who works in the field of geometry is called a geometer. Geometry arose independently in a number of early cultures as a body of practical knowledge concerning lengths, areas, and volumes, with elements of formal mathematical science emerging in the West as early as Thales (6th century BC). By the 3rd century BC, geometry was put into an axiomatic form by Euclid, whose treatment—Euclidean geometry—set a standard for many centuries to follow. Archimedes developed ingenious techniques for calculating areas and volumes, in many ways anticipating modern integral calculus. The field of astronomy, especially as it relates to mapping the positions of stars and planets on the celestial sphere and describing the relationship between movements of celestial bodies, served as an important source of geometric problems during the next one and a half millennia. In the classical world, both geometry and astronomy were considered to be part of the Quadrivium, a subset of the seven liberal arts considered essential for a free citizen to master.