Definition
Y-axis
The y-axis is the vertical coordinate axis; every point on it has x-coordinate 0.
What it means
The y-axis runs up and down through 0. After moving to the x-coordinate, the y-coordinate tells how far to move up or down, parallel to this axis. On a graph, its label and scale tell what the vertical values mean.
Remember it
y is the lift: it carries the point up or down.
Example
Point B is (0, −3). Its x-coordinate is 0, so B sits on the y-axis, three units below the origin.
Watch for
A vertical line is not automatically the y-axis; it must be the named vertical axis of a coordinate system or graph.
Common mix-up
The y-axis is the whole line. A y-coordinate is one number telling a point’s vertical position.
Exact meaning
In a Cartesian coordinate plane, the y-axis is the vertical axis through the origin; every point on it has x-coordinate 0.