ax·is /ˈaksəs/ noun
plural noun: axes
1. imaginary line about which a body rotates. GEOMETRY an imaginary straight line passing through the center of a symmetrical solid, about which a plane figure can be conceived as rotating to generate the solid. an imaginary line which divides something into equal or roughly equal halves, especially in the direction of its greatest length.
2. MATHEMATICS a fixed reference line for the measurement of coordinates.
3. a straight central part in a structure to which other parts are connected.
4. ANATOMY the second cervical vertebra, below the atlas at the top of the backbone.
5. an agreement or alliance between two or more countries that forms a center for an eventual larger grouping of nations.

Origin: late Middle English: from Latin, ‘axle, pivot’.