1. the value of a coordinate on the horizontal axis;

The Collaborative International Dictionary of English v.0.48:

Abscissa \Ab*scis"sa\, n.; E. pl. Abscissas, L. pl. Absciss[ae]. [L., fem. of abscissus, p. p. of absindere to cut of. See Abscind.] (Geom.) One of the elements of reference by which a point, as of a curve, is referred to a system of fixed rectilineal co["o]rdinate axes. Note: When referred to two intersecting axes, one of them called the axis of abscissas, or of X, and the other the axis of ordinates, or of Y, the abscissa of the point is the distance cut off from the axis of X by a line drawn through it and parallel to the axis of Y. When a point in space is referred to three axes having a common intersection, the abscissa may be the distance measured parallel to either of them, from the point to the plane of the other two axes. Abscissas and ordinates taken together are called co["o]rdinates. -- OX or PY is the abscissa of the point P of the curve, OY or PX its ordinate, the intersecting lines OX and OY being the axes of abscissas and ordinates respectively, and the point O their origin. [1913 Webster]WordNet (r) 3.0 (2006):

abscissa n 1: the value of a coordinate on the horizontal axisThe Free On-line Dictionary of Computing (30 December 2018):

abscissaThe horizontal or x coordinate on an (x, y) graph; the input of a function against which the output is plotted. The vertical or y coordinate is the "ordinate". See Cartesian coordinates. (1997-07-08)