Wiktionary
x-axis
n. (context algebraic geometry English) The axis on a graph that is usually drawn left to right and usually shows the range of values of an independent variable.
WordNet
x-axis
n. the horizontal axis in a plane coordinate system
Usage examples of "x-axis".
The more common employs some kind of grid reference west-east, north-south, or on squared graph paper, a horizontal X-axis and a vertical Y-axis.
But the King, nothing daunted, put on his Markov chain mail and all his impervious parameters, took his increment to infinity and dealt the beast a truly Boolean blow, sent it reeling through an x-axis and several brackets--but the beast, prepared for this, lowered its horns and wham!