Definition
X-axis
The x-axis is the horizontal coordinate axis; every point on it has y-coordinate 0.
What it means
The x-axis runs side to side through 0. The x-coordinate tells how far to move along this line before using the y-coordinate to move up or down. On a graph, its label and scale tell what the horizontal values mean.
Remember it
x goes across first; y moves vertically second.
Example
On a coordinate grid, point A is (4, 0). Its y-coordinate is 0, so A sits on the x-axis, four units to the right of the origin.
Watch for
A horizontal line is not automatically the x-axis; it must be the named horizontal axis of a coordinate system or graph.
Common mix-up
The x-axis is the whole line. An x-coordinate is one number telling a point’s horizontal position.
Exact meaning
In a Cartesian coordinate plane, the x-axis is the horizontal axis through the origin; every point on it has y-coordinate 0.