The 
- (horizontal) coordinate of a point in
a two dimensional coordinate system. Physicists and astronomers sometimes use the
term to refer to the axis itself instead of the distance along it.
In older texts, the abscissa is sometimes used in a more general sense as a number determining the position of a point along a line. For instance, a straight line in
Euclidean three-space can be parametrized by 
, where 
with 
. Here, 
is the abscissa of the corresponding point 
on the line.
