Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 noun The coordinate representing the position of a point along a line perpendicular to the yaxis in a plane Cartesian coordinate system.
from The Century Dictionary.
 noun In mathematics: In the conic sections, that part of a transverse axis which lies between its vertex and a perpendicular ordinate to it from a given point of the conic. Thus (fig. 1), in the parabola PAC, AM, the part of the axis AB cut off by the ordinate PM, is the abscissa of the point P. In the system of Cartesian coördinates, a certain line used in determining the position of a point in a plane.
from the GNU version of the Collaborative International Dictionary of English.
 noun (Geom.) One of the elements of reference by which a point, as of a curve, is referred to a system of fixed rectilineal coördinate axes.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun the value of a coordinate on the horizontal axis
Etymologies
from The American Heritage® Dictionary of the English Language, 4th Edition
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Examples

The curve also applies if the abscissa is changed to ‘Amount in Savings’.

The independent  variable coordinate (usually x) of a point on the Cartesian plane is called the abscissa, and the dependentvariable coordinate (usually y) is called the ordi  nate.

One such event is shown on the picture on the right: they infer that an air shower passed through the detector by observing The number of muons seen underground is an excellent estimator of the energy of the primary cosmic ray, as the Kascade collaboration result shown on the left shows (on the abscissa is the logarithm of the energy of the primary cosmic ray, and on the y axis the number of muons per square meter measured by the detector).

When you say “abscissa” your mouth opens sideways.

The problem with matter is that it has dimensions and that the particles are not exactly on the same distance along the curvilinear abscissa within the gravitational field. we will try to mimic a situation where there is no influence on this horizontal plane.
Worldwide Campaign Sheds New Light on Nature's "LHC"  Universe Today 2010

Recently a Lovely new idea has arisen, that global cooling always occurs where Al Gore speaks, since Al usually speaks all over this planet and not too often from L3, this at least the abscissa seems reasonable.
Archive 20081101 EliRabett 2008

The line in the figure represents a distribution, with the abscissa representing “policy” or some aspect of a policy on a numerical scale.

When reading the mathematical psychology textbook, I was puzzled by the fact that all the choice problems were described in terms of gains and losses (actually, almost always gains), whereas the utility functions that were supposed to explain the choices were drawn with wealth as the abscissa.

The raw rock mountains shadowed in the late sun and to the east the shimmering abscissa of the desert plains under a sky where raincurtains hung dark as soot all along the quadrant.
No Country For Old Men McCarthy, Cormac, 1933 2005

Porrò ipsum Paradisi locum audiui à tribus plagis, orientali, meridionali, et septentrionali, inaccessibilem tam hominibus quàm bestijs, eo quòd apparet ripis perpendiculariter abscissa, tanquam inestimabilis altitudinis.
kewpid commented on the word abscissa
The xcoordinate.
September 25, 2007
brtom commented on the word abscissa
Four years on, my personal declaration of, and against, lexical ignorance runs from "abscissa" (Cormac McCarthy) to "zugunruhe" (William Fiennes, who admittedly provided a helpful contextual explanation of this, the migratory restlessness of birds). James Meek
January 3, 2008