In mathematics, a plane is a two-dimensional manifold or surface that is perfectly flat. Informally, it can be thought of as an infinitely vast and infinitesimally thin sheet oriented in some space. Formally, it is an affine space of dimension two.

When working in two-dimensional Euclidean space, the definite article is used, the plane, to refer to the whole space. Many fundamental tasks in geometry, trigonometry, and graphing are performed in two-dimensional space, or in other words, in the plane. A lot of mathematics can be and has been performed in the plane, notably in the areas of geometry, trigonometry, graph theory and graphing.