- the horizontal, or more nearly horizontal, axis in a plane Cartesian coordinate system, along which the abscissa is measured
- a similar axis that is perpendicular to the y-axis and the z-axis of a three-dimensional Cartesian coordinate system
- The horizontal axis of a two-dimensional Cartesian coordinate system.
- One of three axes in a three-dimensional Cartesian coordinate system.
- (algebraic geometry) The axis on a graph that is usually drawn left to right and usually shows the range of values of an independent variable.