axis

A fixed reference line for plotting coordinates on a graph.

"Plot the data points along the x-axis."

A political alliance between countries, often against another group.

"The Axis powers formed a formidable alliance."

An imaginary line around which an object rotates or is symmetrical.

"The Earth rotates on its imaginary axis."

A central line or principal structure in an organism or part.

"The spine forms the body's central axis."