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the line about which a rotating body, such as the earth, turns.
Mathematics.

a central line that bisects a two-dimensional body or figure.
a line about which a three-dimensional body or figure is symmetrical.

Anatomy.

a central or principal structure, about which something turns or is arranged:
the skeletal axis.
the second cervical vertebra.

Botany. the longitudinal support on which organs or parts are arranged; the stem and root; the central line of any body.
Analytic Geometry. any line used as a fixed reference in conjunction with one or more other references for determining the position of a point or of a series of points forming a curve or a surface.
Compare x-axis, y-axis.
Crystallography, crystallographic axis.
Aeronautics. any one of three lines defining the attitude of an airplane, one being generally determined by the direction of forward motion and the other two at right angles to it and to each other.
Fine Arts. an imaginary line, in a given formal structure, about which a form, area, or plane is organized.
an alliance of two or more nations to coordinate their foreign and military policies, and to draw in with them a group of dependent or supporting powers.
the Axis, (in World War II) Germany, Italy, and Japan, often with Bulgaria, Hungary, and Romania.
a principal line of development, movement, direction, etc.
axis deer.
Contemporary Examples

He was convinced that ocean moats would never keep the axis powers out of the Western Hemisphere.
When America Said No to War Marc Wortman September 9, 2013

Wishful thinking is not the best basis for a meaningful alliance, never mind an axis of good.
Obama vs. The World Richard Wolffe September 20, 2009

This could shift global media decision-making from its familiar New York-Los Angeles axis to the Bay Area.
Battle of the Upstarts: Houston vs. San Francisco Bay Joel Kotkin October 4, 2014

And behind the scenes, it seemed, there was almost nothing he would not do to cement the Riyadh-Washington axis.
Prince Bandar bin Sultan, Saudi Arabia’s Gatsby, Master Spy Christopher Dickey November 15, 2013

For Bush, it was branding Iran, Iraq, and North Korea the “axis of evil” in 2002—more than a year before invading Baghdad.
State of the Union Sand Traps Howard Kurtz January 23, 2011

Historical Examples

It lies in the form of a trough with its axis running east and west.
Encyclopaedia Britannica, 11th Edition, Volume 8, Slice 3 Various

The motion of the earth round its axis, and round the sun, makes the day, and the year.
Nature Ralph Waldo Emerson

L is a loop, the axis of which is straight, while R is one the axis of which is curved or crooked.
Dactylography Henry Faulds

Izanagi struck his tall spear in the ground making it the axis of the world.
Japanese Fairy World William Elliot Griffis

The only equable motion she has, is her revolution on her own axis.
The Mosaic History of the Creation of the World Thomas Wood

noun (pl) axes (ˈæksiːz)
a real or imaginary line about which a body, such as an aircraft, can rotate or about which an object, form, composition, or geometrical construction is symmetrical
one of two or three reference lines used in coordinate geometry to locate a point in a plane or in space
(anatomy) the second cervical vertebra Compare atlas (sense 3)
(botany) the main central part of a plant, typically consisting of the stem and root, from which secondary branches and other parts develop
an alliance between a number of states to coordinate their foreign policy
(optics) Also called principal axis. the line of symmetry of an optical system, such as the line passing through the centre of a lens
(geology) an imaginary line along the crest of an anticline or the trough of a syncline
(crystallog) one of three lines passing through the centre of a crystal and used to characterize its symmetry
noun (pl) axises
any of several S Asian deer of the genus Axis, esp A. axis. They typically have a reddish-brown white-spotted coat and slender antlers
noun

the Axis, the alliance of Nazi Germany, Fascist Italy, and Japan, established in 1936 and lasting until their defeat in World War II
(as modifier): the Axis powers

n.

1540s, “imaginary straight line around which a body (such as the Earth) rotates,” from Latin axis “axle, pivot, axis of the earth or sky,” from PIE *aks- “axis” (cf. Old English eax, Old High German ahsa “axle;” Greek axon “axis, axle, wagon;” Sanskrit aksah “an axle, axis, beam of a balance;” Lithuanian aszis “axle”). Figurative sense in world history of “alliance between Germany and Italy” (later extended unetymologically to include Japan) is from 1936. Original reference was to a “Rome-Berlin axis” in central Europe. The word later was used in reference to a London-Washington axis (World War II) and a Moscow-Peking axis (early Cold War).

axis ax·is (āk’sĭs)
n. pl. ax·es (āk’sēz’)

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

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

The second cervical vertebra. Also called epistropheus, vertebra dentata.

An artery that divides into many branches at its origin.

axis
(āk’sĭs)
Plural axes (āk’sēz’)

An imaginary line around which an object rotates. In a rotating sphere, such as the Earth and other planets, the two ends of the axis are called poles. The 23.45° tilt of the Earth’s axis with respect to the plane of its orbit around the Sun causes the Northern and Southern Hemispheres to point toward and away from the Sun at different times of the year, creating seasonal patterns of weather and climate. Other planets in the solar system have widely varying tilts to their axes, ranging from near 0° for Mercury to 177° for Venus.

Mathematics

A line, ray, or line segment with respect to which a figure or object is symmetrical.

A reference line from which distances or angles are measured in a coordinate system, such as the x-axis and y-axis in the Cartesian coordinate system.

Anatomy The second cervical vertebra, which serves as a pivot for the head.

Botany The main stem or central part of a plant or plant part, about which other plant parts, such as branches or leaflets, are arranged.

axial adjective

An imaginary straight line passing through the North Pole, the center of the Earth, and the South Pole. The Earth rotates around this axis.

In geometry, a straight line about which an object may rotate or that divides an object into symmetrical halves.

Note: The axis of the Earth is an imaginary line drawn through the North Pole and the South Pole.