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 axis

The Free On-line Dictionary of Computing (30 December 2018):

abscissa

    The 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)