- (in a plane Cartesian coordinate system) the axis, usually vertical, along which the ordinate is measured and from which the abscissa is measured.
- (in a three-dimensional Cartesian coordinate system) the axis along which values of y are measured and at which both x and z equal zero.
- a reference axis, usually vertical, of a graph or two- or three-dimensional Cartesian coordinate system along which the y- coordinate is measured
