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
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.