1. A straight line about which a body or geometric object rotates or may be conceived to rotate.

2. Mathematics. a. An unlimited line, half-line, or line segment serving to orient a space or a geometric object, especially a line about which the object is symmetric. b. A reference line from which distances or angles are measured in a coordinate system.

3. A center line to which parts of a structure or body may be referred.

4. An imaginary line to which elements of a work of art, such as a picture, are referred for measurement or symmetry.

5. Anatomy. a. The second cervical vertebra on which the head turns. b. Any of various central structures, such as the spinal column, or standard abstract lines used as a positional referent.

6. Botany. The main stem or central part about which organs or plant parts such as branches are arranged.

7. One of three mutually perpendicular lines that define the orientation of an aircraft, with one being along its direction of travel and the other two being perpendicular to the direction of travel.

8. A line through the optical center of a lens that is perpendicular to both its surfaces.

9. One of three or four imaginary lines used to define the faces of a crystal and the position of its atoms.

10. a. An alliance of powers, such as nations, to promote mutual interests and policies. b. Axis The alliance of Germany and Italy in 1936, later including Japan and other nations, that opposed the Allies in World War II.