Pronunciation: (eks'ak"sis),[key] —n., —pl.x-ax•esPronunciation: (eks'ak"sēz).[key]Math. 1. Also called axis of abscissas. (in a plane Cartesian coordinate system) the axis, usually horizontal, along which the abscissa is measured and from which the ordinate is measured. See diag. under abscissa. 2. (in a three-dimensional Cartesian coordinate system) the axis along which values of x are measured and at which both y and z equal zero.