abscissae [-si:] or abscissas
The English word 'abscissa' means the value of a coordinate on the horizontal axis
The coordinate representing the position of a point along a line perpendicular to the y-axis in a plane Cartesian coordinate system.
The abscissa is the x or horizontal coordinate. The ordinate is the y or vertical coordinate. I remember them because they are both alphabetical.
It's x = 0. Consider a point of the plane, P=(x, y), in cartesian coordinates. If P is a point belonging to x-axis, then P=(x, y=0); if P is a point belonging to y-axis, then P=(x=0, y).