Definition
Axis
An axis is a fixed reference line used to locate positions, read values, or describe symmetry or rotation.
What it means
Think of an axis as a line that gives the rest of a picture a reliable starting direction. On a graph, labels or numbers are placed along it so you can tell what a point or bar means.
Remember it
Find the reference line first; then read what is placed along it.
Example
A bar graph shows fruit names along the horizontal axis and numbers 0, 2, 4, 6, and 8 along the vertical axis. A bar reaching 6 means that fruit has a value of 6.
Watch for
A grid line is not automatically an axis. The axis is the selected reference line that carries the coordinate or graph meaning.
Common mix-up
The horizontal axis is not always a number scale; in a bar graph it may carry category names.
Exact meaning
An axis is a chosen reference line that organises positions or values. Coordinate grids use perpendicular axes to locate points, graphs place categories or quantities along axes, and other geometry contexts may use an axis of symmetry or an axis of rotation.